You can't bring yourself to pull out, much as you know by common sense you should. Your cock is rubbing against her G-spot, so she's clinging tightly to you, close to her own orgasm. And just to make it impossible, you feel her legs wrap around your back, holding your hips in place and making it so there's only one place you can finish.
Groaning in deep tones, your body quakes and your cock head and her vaginal walls both pulse rhythmically as you cum, your cock head twitching as you shoot jet after jet of your cum inside her. For a wonderful moment, all civilized context is unimportant and you both feel intense satisfaction at successfully mating.
This quickly gives way to intense concern that you've just gotten a girl knocked up, and when the teacher comes round to look at you and notices your bodies are both entwined in a mating press, he tuts.
Once you've recovered, the teacher reads off a list of names of people who came inside, kinda contemptuously, and your anxieties only get worse. Are you gonna have to pay for her to raise the child or something?!
Thankfully, when he announces what you've got to do, it's not that. But it is something almost as unbelievable as what you'd just been doing in this weird sex PE lesson...