Omocha wa 27-sai jokyoshi

The teacher decided to improve the performance of her student by making him an offer from which he could not refuse: if he becomes the best one in the whole school, he can do whatever he wants with her. And he became still, now sending all his curiosity and imagination to his beloved teacher.
However, at some point, all three students will know about everything, who themselves are not averse to experimenting.