Mesu Kyoushi: Injoku no Kyoushitsu 3

Based on a game by Bishop.

Toudou Miyuki and Sanada Eiji are teachers in the same school and lovers. They had decided that they would preserve their chastity until they are married. However, Miyuki is assaulted and loses her virginity to Takuya, Eiji's step-brother. Furthermore, it doesn't stop there as Takuya turns his attention on the other female teachers one after the other.