Posted originally on the Archive_of_Our_Own at https://archiveofourown.org/
works/2400692.
  Rating:
      Explicit
  Archive Warning:
      Underage
  Category:
      M/M
  Fandom:
      Supernatural
  Relationship:
      Castiel/Dean_Winchester
  Character:
      Castiel_(Supernatural), Dean_Winchester
  Additional Tags:
      Alternate_Universe, alternate_universe_-_AU, Alternate_Universe_-_High
      School, teacher!Cas, Student!Dean, Underaged!Dean, Sub!Dean, Dom!Cas,
      bottom!Dean, Top!Cas, BDSM, Dom/sub, Butt_Plugs, Spanking, Vibrators,
      Math_Kink, Maths_(sorry), Romance_(Kind_of), Fluff_and_Smut, PWP_without
      Porn, PWP, Porn_Without_Plot, Plot_What_Plot, Smut, Shameless_Smut,
      Orgasm_Delay/Denial, Dirty_Talk, Castiel_Talks_Dirty, Anal_Sex, Anal_Plug
  Series:
      Part 3 of Math_is_Fundamental
  Stats:
      Published: 2014-10-04 Words: 2963
****** Some Concepts Are Better Understood Naked ******
by Just_Breezy
Summary
     With Dean's new relationship with his calculus teacher, his grades
     have been slipping, and without any explanation, Bobby and Ellen did
     what any (albeit surrogate) parents would do: ask Mr. Novak to give
     Dean one-on-one tutoring sessions.
Notes
     So, if you've been reading the 'series', I'm sure you've picked up on
     the fact that I may have a certain... affinity to math, but I loathe
     logarithms and was going to skip over this unit. However, we're
     running out of units to cover before Dean graduates, so I'm sorry if
     there are any problems with the actual math
     Also, as ever, you don't *need* to read the series to appreciate
     this, or any of the fics in the 'series', they're all good as simple
     porn.
     Personally, I still prefer Illegal to this, but you know the deal,
     enjoy the smut!
      
     OH! Be warned: There's romance at the end of the fic, it's grossly
     sweet and slightly OOC.
    There are many things that come naturally to Dean, but even he need to pay
attention in calculus. Which he hasn't. With his grades starting to slip and
neither Bobby or Ellen could understand why. So they did what any concerned
(albeit surrogate) parents would do: ask Mr. Novak to give Dean private
tutoring sessions; and Mr. Novak being the benevolent and worried teacher he
is, agreed.
    As anticipated (by Dean), the sessions have gone from very informative and
productive to this:
Dean sat in the cool chair, occasionally losing focus on the matters at hand to
address how cold the plastic felt against his naked body, or how the cuff
linking him to the chair/desk rattled as hr wrote down notes, trying to keep up
with the information. The real deterrent being Castiel himself, or rather the
fact that all he was wearing was an intricate set of leather straps reaching
over his shoulders, waist and down to his throbbing cock, all meeting in a ring
just above his navel. The guy could keep a boner even when 'teaching' math, and
though impressive, very distracting.
    "Mr. Winchester, I don't believe the problem will solve itself." Dean
focused on the board, finally averting his eyes from Castiel's firm leather-
clad form. The question read 'Find the derivative of f(x) = log5(xex)'.
Thankfully, Dean wasn't paying attention, or really understood how to
differentiate logarithms.
    Of course, if Dean told Mr. Novak that he wasn't paying attention, the
punishment would be far worse than simply getting the question wrong. As he was
writing out a completely guesswork answer, Mr. Novak became impatient, "hurry
up Mr. Winchester, we have a lot to cover." Dean scribbled down the last of his
answer and raised his hand to signal his completion.
    
    "And the answer is?"
    "xexln(5), sir."

    Dean spasmed violently while breathy moans coated his lips as Mr. Novak
switched the vibrating plug from 0 to 3. Dean had long since adjusted to the
feeling of it, but every time he got a question wrong, Castiel would make it
vibrate viciously, sending his student into a rapture of pleasure and shock.
Castiel gave Dean three minutes of the vibrator on it's highest setting before
finally turning it off. Dean felt entirely shaken and raw from the
overstimulation, but wickedly turned on and desperately unsatisfied. Castiel
had been getting creative with his punishments for his petulant student; it
seemed sexual stimulation worked better than simple force.
    "Tell me, boy, where do you think you went wrong?" His student shivered,
knowing if he got this question wrong, Mr. Novak wouldn't hold back on the
controls. If anything, he enjoyed it more when Dean got questions wrong. "Um,
maybe-" his words were consumed by a shuddering moan as Castiel twisted the
dial. "We've talked about indecisiveness, Dean. You cannot think the answer
might be right, either it is or it isn't, there is no in between." The older
man's smoky voice affected Dean more than the vibrating plastic against his
prostate. No matter what Castiel was saying, it always turned Dean on,
especially when he was reprimanding Dean, or praising him.
    The boy continued to writhe in his chair while Castiel mercilessly carried
on the onslaught, "I'll ask you again, slut, where do you think you went
wrong?" Castiel leaned against his desk at the front of the class casually,
while Dean wantonly mewled in a desk in the second row, blue chair warming up
under him as his body heated up, blush painting his freckled skin.
    "Wh-when I applied the- natural logarithm rule," Dean moaned around every
word, barely able to get the sentence out. Castiel cocked his eyebrow, seeing
Dean would be nearing his orgasm at the rate he was being stimulated. Castiel's
own erection desperately ignored and heavy between his legs. Deciding the only
appropriate punishment for Dean's blatant distrait behavior, Castiel chose to
enjoy the show and wait until his student was on the verge of release.
    The boy's eyes rolled back into his head while the continued vibration
became less overstimulating and more pleasurable as Mr. Novak turned the dial
down to a much better setting, his teacher stroking his cock at the beautiful
sight of his little slut melting into the touch of impersonal plastic. "You're
so desperate to have your ass nice and full, you would come from a plastic plug
shoved up your ass, you don't even care what it is, as long as it fills you
up," Castiel noted mostly to himself, as Dean was preoccupied with getting very
close to coming, canting his hips on the chair. "Go ahead, pet, you can touch
yourself," Mr. Novak murmured just loud enough for Dean to hear. His shoulders
relaxed as his hand found his dreadfully neglected cock, his teacher quietly
moaning with the boy; "that's it, just enjoy it," Castiel continued to coo.
    Dean was in complete bliss an so very very close. Hs mouth went slack as he
felt the familiar coil in his stomach about to release- then nothing! Mr. Novak
turned the plug off, jolting his student out of his nirvana. "You may remove
your hand from your cock, Mr. Winchester, we have a question to work through."
Dean's eyes flashed open, shock and indignation evident within the brilliant
green of his eyes, jaw hanging open for completely different reasons now.
Castiel grinned sinfully watching Dean's stunned face while his hand
instinctively flew off his dick; Castiel was certainly doing a fantastic job of
making his perfect slut the perfect sub. Dean searched his teacher's face for
any indication of why he wasn't allowed to come, or why he was given so much
indulgence only to have it taken away so quickly, but found only sadistic
satisfaction. Mr. Novak nonchalantly stroked his cock with one hand while
writing on the board with the other, clearly relishing in his student's
discontent. Castiel love his pet's face in ecstasy, but only when he was the
cause, otherwise it was much more gratifying to watch him be left unsatisfied,
the best punishment.
    "So, as we can see, the answer is f'(x) = (x+1)/xln(5)." Castiel turned
around to his student frantically writing down the equatoin, still flushed from
the almost-orgasm. Mr. Novak walked to Dean's desk, smiling lasciviously, and
knelt beside his young student, "my poor student," Castiel began to run his
hands up Dean's thighs, tracing patterns into the sensitive skin, "you're
working so hard," Castiel began kissing his chest, stopping to suck and bite at
his student's nipples, "and I don't even let you come." Castiel's hands began a
torturously slow pace on Dean's hard on, while his mouth continued to explore
his chest and neck, occasionally the leather he wore rubbed against his
student's responsive body, causing Dean to shudder and moan. Castiel continued
to hum words of praise into his pliant student's neck, reverberating throughout
Dean's body, setting every nerve on fire; he loved and hated the excruciating
drag of leisurely  pleasure, but he also knew that was the intention, and that
seemed to make it all the more better and painful.
    "Can my favorite slut answer one simple question?" Castiel purred. "Of
course, sir." The older man smiled, remembering the dauntless boy he began
with, now so very obedient. "Can my beautiful pet tell me the derivative
of ecot^-1(x^2) + x3?" Castiel turned the plug on low as Dean began to work,
only momentarily distracting the boy. Dean shifted lightly as Castiel began
kissing from his knee moving upward, "I can't decide what I love more: your
face when working through a problem, or your face right before coming. You are
truly beautiful, Dean." The last of the sentence was said around the head of
Dean's cock; the boy's mouth formed a perfect 'O', delicate lips parting in
undiluted bliss, while his eyes remained completely focused on the task at
hand. Castiel smirked and the dial up, hoping to break his young student. Dean
stopped and enjoyed the delicious stimulation, even if it drove him mad,
needing to come, like yesterday. He continued his work, hand unconsciously
moving to his dick as his not-so-benevolent teacher removed his mouth. This
earned him a harsh slap, "I'm disappointed, Mr. Winchester, there aren't many
rules, and yet you still seem to struggle." Dean looked wide-eyed at Castiel,
completely haulting his work, "please sir-" he began. "Finish the question,
then we shall discuss appropriate actions."
    Dean finished the question hastily, raising his hand again. 

    "And the answer is?"
    "f'(x) = ecot^-1(x^2) [- (1/(x2)2+ 1) (2x)] + 3x2], sir."
    "Excellent work, pet. You've been very good, but you've also broken a rule,
and you know what that means, and for that I believe an indefinite number of
spanks is appropriate, while for all your hard work, you can request one thing.
Does that seem fair, slut?"
    "Thank you, sir! I- yes, that seems fair... can you- please fuck me? This
plug isn't you, and I just-! Please let me come on your cock, sir!"

    Castiel smiled, god, Dean was a cockslut and especially needy after a long
tutoring session, and endlessly gorgeous when desperately trying to satisfy
himself without having any contact, (beyond the relentless vibrating plug).
"Very well then, Mr. Winchester. Lets begin with the spanking." Dean
intuitively got up from his seat, and though he was cuffed to the arm,
manoeuvred his way around it until his pert ass writhed in the air, on display
and waiting, the black plug sharply contrasting his pale skin.
    Mr. Novak walked behind the boy, erection gently teasing his thigh as the
older man leaned over to whisper into the boy's ear, "I'm not going to be
gentle." With that, Dean trembled with exhilaration as Mr. Novak's hand made
harsh contact with the boy's ass. Both of them winced with enjoyment for very
different reasons. Four more slaps followed in quick succession. Each time
Castiel's hand made contact, the older man would turn the dial up slightly,
ensuring every now and then his hits touched the teen's sensitive rim,
enhancing both the pain and pleasure equally. Castiel hit harder, keeping true
to his word, ensuring Dean was a hot mess, writhing and wordlessly begging on
the impersonal desk. Castiel loved watching Dean react so eagerly, especially
while submitting so wholly to the punishment. "Do you think you've been scolded
enough, pet?" The older man rubbed affable patterns over the smatterings of
forming handprints, humming quietly while Dean collected his thoughts. "N-no,
sir." His teacher raised an eyebrow as Dean canted his hips towards the older
man. "Why's that, Mr. Winchester?" Dean's legs trembled, threatening to give
out, matching the fluttering of his voice, "I sho-should know better by now
than to br-break a rule."
    Evidently, Dean was quickly becoming the perfect sub, only making Castiel
love him more, "very well then." Castiel continued to revile the teen's
reddening ass until he felt the boy could not take any more. The teen panted
while his teacher knelt between his lissome student's overworked legs, kissing
the ridges of forming welts. "Now, lets get this thing out of you," Castiel
lulled, fingers pressing against the sensile ring of muscle holding in the
black vibrator. The student gasped as Castiel pulled the plug out forcibly.
With the plug gone, Dean mewled at the emptiness. Castiel kissed the boy's ass
before standing up and patting him on the back, "turn around and sit on the
desk, slut," Castiel commanded.
    Without wasting a moment, Dean stood up and shuffled awkwardly, right wrist
still cuffed to the arm, until he was sitting on the desk, facing Mr. Novak,
hands begging to hold onto his master's leather straps, while his legs
instinctively opened wider. Castiel reached under Dean's seat, grabbing the
lube from his student's pants, and began rubbing some of the cool liquid over
his aching cock, watching Dean shift excitedly on the desk, waiting.
    The older man grabbed the student's legs from behind his knees. Dean leaned
back, looking for any indication of what to do next, anticipation building.
Their tutoring session had been going on for almost two hours, and finally,
there would be glorious relief. Dean's legs rested on the black leather straps
covering Castiel's shoulders, giving his calfs a perfect bite of leather.
    Castiel pushed in unceremoniously into the lithe teen, pressing the hard
leather over his perineum, relentlessly exploiting the teen's overstimulated
prostate on the first thrust. The older man savoured the delicious sounds
coming from his student. Setting up a ruthless pace, Castiel fucked Dean for
all his worth, fragile legs shaking on his shoulders, exquisite moans a
continuos mantra. With every astringent thrust, Dean was treated to a strdent
slap of leather as well, making the pain and pleasure that much more consuming.
    
    "Are you owned!?"
    "Yes-!"
    "Are you my bitch, Dean!?"
    "Oh god, yes, I'm your- your-!"

    Castiel stalled his movements, still completely buried within Dean, "tell
me, boy, will you ever be able to fuck another man?" Dean tried to thrust
himself on Castiel's dick to no avail. "N-no, please sir, fuck-!" The older man
smiled, "then why aren;t you coming?" With that, Dean let go without even
realizing he was holding back, screaming, his body moved of it's own accord,
writhing on the desk. Castiel began fucking Dean through his orgasm, not
lasting long as the boy tightened around his cock, throbbing over the length.
    Dean's legs went lax and slid off his teacher's shoulders, completely
spent. Castiel slipped out the teen and grabbed some tissue from the dispenser
by the door along with the keys to the cuffs.
    The boy watched as his teacher wiped up his slack abdomen and undid the
cuff from his wrist. Castiel moved to his plush chair at the front of the room,
motioning for Dean to come, "aren't you going to join me?" Unwilling to move,
Dean got up, crawling onto the older man's lap, face snug against his neck,
legs falling off the side of the chair. Castiel hummed in satisfied ease as he
rubbed Dean's muscled back, dropping kisses to his student's head every now and
again.
    
    "Dean, I have to ask, as your teacher, did you actually learn anything
today?"
    "Of course. I learned that you need to wear this way more often, quite
possibly under your ridiculous sweater vests."

    Castiel frowned and lightly smacked Dean's shoulder, "I'm serious, should I
look into tutoring options?" Dean sighed, he really didn't want to be taught
math by anyone but Castiel; "maybe I'll try actually paying attention in class
before we start looking at tutors." Castiel settled back into the cuddle,
teaching duty done, loving the warmth of Dean's skin against his, though he
could still feel his student was not as relaxed.
    "What's wrong, Dean?" The boy entirely locked up before forcing himself to
be completely lax; "nothing's wrong..." Castiel frowned again and pulled away
slightly to look at Dean better, "seriously, what's on your mind?"
    Dean shifted uncomfortably before standing up and walking back to his desk,
where his clothes were. The boy tried to hide how red his face was, a mix of
emotions contorted his graceful features. Castiel watched, nervous with what
was to follow. The boy dug through his pants before returning to his teacher's
lap, hands clasped around something, sending Castiel's heart rate through the
roof. "Dean, please tell me that's not-" The boy opened his hands, presenting a
small black velvet box.
    "No, shut up. This is... god, this is so lame... I hope... I hope you
appreciate that I'm breaking my 'no chick flick moments' rule. Fuck, alright,
Cas, first of all, this is not an engagement ring... this is my promise to you
that... fuck, I'm sorry, this is all new to me... but I know our whole
relationship is fucked up, but I want to make this work, I want this to last,
because... well, fuck, you just, you make me so happy, and... I don't want that
to end anytime soon."
    Dean stared at the box the whole time, unwilling to see the possible
mockery in Castiel's face, before opening the small box to reveal a thin,
simple silver ring. Castiel couldn't breathe, face showing the apparent awe.
The teen finally looked up to see Castiel staring back at him, both lost for
words.
    Dean began to panic as Castiel didn't immediately respond, when finally, as
Dean began to pull away in horror, the older man wrapped his arms around him
and kissed him passionately, pushing the tears of happiness back. He was not
going to cry that his student, his boyfriend felt just as strongly about him as
he did.
    The teen pulled away from the kiss, gasping for air, finally breaking into
laughter (and secretly extreme relief).

    "So, I'll take that as a 'you accept'?"
    "Of course, you assbutt."
    "'Assbutt'? Really?"
    "I'm sorry I'm not as articulate as you, Mr. I-know-our-whole-relationship-
is-fucked-up."

    They both laughed and Dean slipped the ring onto Castiel's right ring
finger (as much as he wanted Cas to wear it liked a wedding band, that would be
far too suspicious for a single teacher), kissing the digit once it was on.
Both men admired it, fitting perfectly on Castiel's finger. "I love it, Dean, I
love you... but next time you mysteriously pull out a velvet box, there better
be a diamond ring in there."
    Dean and Castiel laughed and breathed in the air of love and the perfectly
awkward and un-serious expression of love. Somehow, if it wasn't weird,
uncoordinated and bordering on hilarious, it wouldn't be them... and neither of
them would have it any other way... 
Please drop_by_the_archive_and_comment to let the author know if you enjoyed
their work!