Sometime during the lesson, a piece of balled-up paper was thrown at your head. Unfolding it, you found a message:
"Rooftop after class. Be there."
The tone of the message conveyed a sense that it was to be obeyed. Even though you were looking forward to lunch (you had to pass on breakfast this morning because the fridge broke down), you went right up to the rooftop as you were commanded. No one was there and for a while, you thought that maybe this was some kind of prank.
But then someone swung out from behind the HVAC unit. It was big, red, and bulging with muscles.
"You came." Damian stood before you, not only completely nude, but also noticeably older. He must have seen you trying to put the pieces together, because before you could ask, he answered.
"I'm not really what you'd call a student, mortal. Or at least, I am a student, but I'm really here because I intend to make this school my kingdom on Earth. You're here because I happen to need a few helpers."
With that, he jumped down to present his cock, which began to swell and grow until it was the same thickness as a baseball bat. His long tail happily slapped around as he got more and more erect, his throbbing member pointed right at you like an arrow.
"Now then, take of my seed, mortal."
To make sure all hesitation was removed, he looked right into your eyes with his own golden irises, so piercing and focused they made any impulse you had to run, to refuse, to escape, disappear. You stripped naked and picked up Damian's big cock with your hands. He grunted as you wrapped your mouth around it.
"That's right, my child. Suck it." He gently grabbed your head and pushed it down.
Time seemed to stand still as you gave this demon the blowjob of a lifetime. He groaned, his cock started to pulse, and then, without warning, he came. His semen went down quickly and smoothly into your throat.
First, your skin started to turn the same shade of red as his. Then your tailbone burst out into a leathery tail, not as long as his but still very impressive. Then a pair of wings emerged from your back, horns from your forehead. Your cock swelled until it was the same thickness as his. Again, it was impressive, but not as impressive as his.
Damian smiled as he pulled out. "From now on, you will be the eyes that guide me, the tongue that whispers into the ears of the weak and pitiless, the cock that fills others with your seed as I have now. Submit before me!!!"
Bowing before him on your knees, you no longer thought of anything besides serving the Lord of Hell. Taking stock of your naked demon body, you found it very...liberating. Taking flight, you masturbated with your new cock and sprayed thick semen all over the rooftop.
"Come here, my servant. Let's put you in a more appropriate form. Something of your choosing, of course."