Ryoujoku Gakuen Cheer Girls Koukai Choukyou

Kamuro High School is famous for its great sports achievements.
Main character comes here as a part-time teacher and uses zero active members club advisor role for voyeurism. One day director appoints protagonist to deal with the incident happened to the cheerleader club member. That unveils an underground school life full of sexual slavery.

[From vndbreview]