In the past a warlock put a curse on Haruma high school, and students became lazy and spent days in a corrupt manner. The school guardian Aoi sealed the warlock and returned peace to the high school.
50 years from that time the seal got weakened and the warlock broke free. However, he lacks the power and starts to use maiden girls to renew powers.
Only main character can revive Aoi, but he also can't allow warlock full resurrection. To prevent that he must seal maiden girls with the power of sex.

[From vndbreview]