Tsugunai 2 ~Mou Hitori no Nana~

Main character Ryoichi teaches at school and lives with his sister Nana. One day his sister does not come home from school, and she is found drugged and violated. A woman appears before protagonist and claims that head of powerful corporation stands behind assault on the girl. Protagonist knows that president's daughters study at his school, so he starts preparations to have revenge over these girls.

[From vndbreview]