Sensei no Okiniiri!(2002)

Sensei no Okiniiri!

Misuzu just started her dream job teaching at a prestigeous high school, and she feels totally motivated. Recently, she began going out with the principal Kazuki, and their relationship couldn't be better. But one day. Kazuki's younger brother Masahiro, who happens to be a student at the same school, forced Misuzu (against her will) to have sex with him. But, somehow, it turns into love?! This is a huge secret that they can't tell anyone: the love affair between a teacher and a student.

(Source: Shoujo Magic)