Hi.Me.Go.To

A boy and a girl are riding the train to school. They are a couple, but they haven't had sex yet because he was unable to perform when they first tried. When he sees his teacher being groped on the train, he feels a new sensation. When his teacher realizes that he was watching, she slowly transforms him into her ideal partner - on the train or in the hidden corners of the school. While his girlfriend feels abandoned by the absence of his presence, he is there having sex with his teacher and being trained to be a better lover. But when his teacher's car is fixed and she no longer has to ride the train, the relationship ends, he goes back to his girlfriend and this time, he is able to perform.

(Source: Anime News Network)