You Taught Me All the Precious Things (2011)

Original title: 大切なことはすべて君が教えてくれた

Shuji is a young teacher whose confidence and honesty has made him popular with the students. He is engaged to Natsumi, also a teacher at the same high school. At the morning of the new semester, Shuji wakes up with a hangover and no memory of the night before, and a strange young woman in his bed. He feels terrible about possibility of having cheated on his fiancée, but when it turns out the girl is a student in his class Shuji is wrecked with guilt. Things go from bad to worse when the girl, Hikari, makes it clear that she's in love with him and wants to break him and Natsumi apart. Shuji's honest nature makes him want to come clean to everyone about what has happened, but before he has the chance, the secret is out. As his life starts crumbling around him, Shuji still decides to try and save Hikari from herself.