せいぎのみかた～ただいま着替え中！～

Main character works as social studies teacher. He has amnesia and only remembers last three years of his life from the moment of lying on operation table in evil organization headquarters. He was implanted a compact bomb into his head during that time to force cooperation.
There is a self-proclaimed female organization of justice "Astrea" at school. It's only a matter of time when two organizations clash.

[From vndbreview]