Big Magnum Harimoto-sensei

Main character is dispatched to investigate mysterious secret society that hides inside St. Eric's high school. The granddaughter of the director, Elina, notices that she is being targeted by a secret society. Protagonist protects her, and gradually relations start to develop. Eventually Elina gets kidnapped in order to get some "treasure" sleeping in the school. Who will be victorious?

[From vndbreview]