Oshioki: Gakuen Reijou Kousei Keikaku (Discipline Curriculum)

Synopsis

My name is Shimohei Joutarou. There is a reason that I'm here, at my new place of work, despite the fact that the school year hasn't started. On that first day, I almost immediately came across Mitsuki's and Minami's secret. I then heard about Mahiru, younger sister of Classics teacher Kuji Hinako, and the astonishing improvement of her grades. I then discover indications of Hinako's backdoor maneuverings. Now, it is up to me to rehabilitate these students who break the rules. (Source: translated and adapted from official site by Cranston)