A documentary starts up, and it's one about hermaphroditism. You've covered this a little in biology before- hermaphrodites are animals that have both masculine and feminine reproductive organs. It shows a few examples you've covered- earthworms, toads, clownfish- but you notice something weird. The narration is turning you on. You blush as you realize you have an erection.
For a moment, you feel deeply worried, but then you hear a rustling and look to your left- the boy next to you has his pants unzipped and is jacking off! You look right and see the girl next to you has a hand under her top and another between her legs! Practically the whole class was playing with themselves, and everyone felt horny. The only way you could even try to rationalize it was that maybe it was all the talk of genitals.
Next it does something that takes you, and basically everyone in class, by surprise. It shows renderings of a naked male and female human. "Let's do a quick reminder, just so no one gets confused," the narrator says in its androgynous, almost sensual voice. "There are two types of hermaphrodite: protandrous and protogynous."
It shifts to focus on the male human. "To make this a bit easier to remember, here's a protandrous human." It zooms in on his crotch, and you feel something deeply weird between your own legs. "As he comes to sexual maturity, his penis shrinks, his testicles retract and he loses the capacity to fertilize others." You look down, undo your pants and gasp in a higher-pitched tone as your own dick and balls have started to recede away! Instead of hurting or even feeling uncomfortable, it just feels really pleasurable.
"Once he's emasculated, his mammaries develop rapidly," continues the voice. Your chest feels oddly sensitive, and you pull off your shirt to see your nipples have perked up and they swell and widen before your eyes. Almost in sync with the young man on the screen, you start to sprout breasts, which hang from your chest and ache to be touched in their aroused state.
"Finally, his- or should I say her?- crotch fully develops into a vagina," the voice concludes. A sucking feeling seems to emerge inside your body. "Inside her, the testicles have reshaped into ovaries, converted to holding ova rather than sperm, and connected to a womb. All that remains is for the vaginal opening to form so the body can be impregnated." The little sensitive nub that your cock has become settles in above the by now flat mount that had been your scrotum, and your body opens out, a vaginal slit emerging between your legs.
The camera moves to the female human. "Now to show you a protogynous human," says the voice. "Once she reaches sexual maturity, her mammaries recede away as her reproductive role adjusts." You could see the students who had not just grown breasts were starting to lose them.
"Down below, her vaginal slit seals itself up and her ovaries start to transform and move into the new sac that forms from it." The girls grunt in deeper voices, seeming to push at something, until they relax as the process finishes.
"Now the body is producing semen and sperm. Instead of being able to be impregnated, her- or rather his- body can now fertilize others with its seed. All that's left is to develop the clitoral nub into a phallus." Around you, a protrusion emerges from every former girl's crotch until they have penises of about 6 to 7 inches.
"In other words, protandrous is male to female, protogynous is female to male. Of course, the other notable thing is how intense the arousal response of the body's new formed reproductive system is. You can discover that for yourself..." As you look at the mostly naked transformed people in your class, you realize that's pretty accurate...