Mesu Kyoushi: Injoku no Kyoushitsu

Based on the erotic game by Bishop.

Takuya's parents divorced when he was a child. His mother took him and she got married to another man, Sanada, soon. Sanada's son, Eiji, was a genius and looked down on Takuya. Takuya couldn't stand it anymore and left his house. He entered a high school with his mother's family name. One day, there comes a new teacher to his school. It is Eiji, his older brother. Accidentally, Takuya sees Eiji and Miyuki, a popular female teacher, meet secretly in a vacant classroom. He decides to take a revenge on Eiji and plots to shag with Miyuki in cooperation with Rihoko, a school doctor...

(Source: ErogeShop)