Most of the people who didn't cum inside get led away, off to some other sexual lesson. You, your partner and all the other pairs who came inside are kept there, and you're increasingly confused by what you're all gonna have to do. Do you just have to sit here and have naked detention or something?
"Right," the teacher announces. "Since all of you pairs didn't think to use a condom or pull out, we're gonna do a little role reversal. The point is to show boys why you shouldn't carelessly cum inside, and to show girls why boys might find their instincts hard to keep control of during sex."
One of the boys puts his hand up. "Wait, so how are we doing a role reversal?"
"Like this!" the teacher replies. He picks up some weird gadget and presses a button on it, and a bright light flashes over everyone in the room for a moment.
You feel a weird shiver run through you, and look at Mina. "Did you feel that?" you ask.
"Yeah, it felt kinda... mmmh..." she murmurs, another chill racing through her.
"I feel kinda... ahhh..." you reply, as you feel the weird feeling pulse through you again too. You suddenly notice a warmth in your chest, and before your eyes your nipples seem to expand. Looking at Mina, you see her breasts have started to shrink, and to your surprise you find her more toned, masculine body kinda... hot.
You're realizing your bodies are changing in parallel to each other. So this is what the 'role reversal' was gonna be- you had to had sex as a girl, and Mina as a boy! You watched entranced as her... his?... clit swelled up and his pussy fused into a scrotum, while your cock rapidly shortened until you only had a small nub between your legs and your balls were sucked up inside you with a flat slit forming from your scrotum, a vaginal passage forming inside you. By the time his breasts had vanished into his firm chest and you had spawned soft B-cups for him to play with, any nonsexual thoughts had been pushed to the back of your mind. You wanted his cock in you, and he wanted to be inside you.
Lying back for him and spreading your legs, he got atop you and positioned his cock on your tummy, slowly sliding it down and rubbing along your newly-formed moist lips, just to make you ache for him before he pushed inside. Once he did, you were amazed at how BIG his shaft felt. Understanding your feelings, he softly kissed your head and reached to cup one of your boobs and play with your nipple. It felt great, and you started to secrete loads of female juices around his shaft, loosening your bodies up and lubricating things.
He looks down at you and echoes your words: "I really wanna move."
Panting, you reply, "I think you can do it now," and as he starts thrusting things quickly start to feel a lot more intense. His body slaps against yours forcefully, and you feel every touch of his flesh against yours- his hands holding your wrists down, his hips brushing against your thighs, his balls slapping your pussy, and most of all his seven inch shaft filling your slit and rubbing along the sensitive G-spot inside.
Even thinking about what's happened is weirdly pleasurable to you. Remembering how you've been forcibly emasculated and given a feminine body to be fucked is oddly exhilarating to you, and being fucked by the boy who used to be Mina doesn't feel at all wrong. Seeing a man thrust and fuck above you, and your own breasts (oh fuck, you have breasts!) bounce and jiggle is amazingly hot.
At last you feel him pulse, and realize there's no way he's going to pull out of you- nor does your body want him to. You hear him grunt and feel him press against your thighs, his cock twitching as it starts to shoot thick cum deep inside you. The first feeling of his moistness touching your inner depths pushes you to your peak; you groan, your eyes squeeze shut and you feel your nipples and clitoris, already intensely erect, seem to pulse with pleasure in time with his cock squirting his essence inside you.
You both feel intense primal satisfaction in the afterglow, though it's reversed with your sexes. Instead of the satisfaction of filling another with your seed, passing on your genetics, you feel the satisfaction of being fertilized and made able to bear young.
Leaning up, you kiss Mina- or whatever his choice of a boy's name is- messily, and reality slowly irises in on you both...