Taiiku Kyoushi Kiwame

Essentially, the protagonist receives an anonymous mail with a dirty mag w/ his gym teacher in it. He uses it to blackmail his teacher.

Then day by day you choose approaches to use on the teach (+other playable charas), like feeling him up or mock him, etc.

If your combination of actions is successful, you'll get FG's trademark interactive H scene where you click on the chara to remove his clothes and make him moan.