Yokujou Kyoushi

At Shinyuu Academy, the protagonist, Inui Ken, is an average student who doesn’t stand out much. But he’s been thoroughly enjoying his school life—thanks to the three beautiful teachers: Yuuho Akiyama (Japanese literature), Harumura Kaoru (mathematics), and Natsuki Kanako (physical education).
Every day, Ken takes advantage of the little glimpses he catches during class—the outline of their underwear, the curves of their bodies—letting his dirty imagination run wild. But one day, after school, one of the teachers corners him.

Will Ken’s fantasies become reality, or will things take an unexpected turn…?

[Translated from dlsite]