Kando Houkai! Kime Seku Seito Kaichou - Saiinzai ni Oboreru Gakuen

The head of the student council Mikako is worrying about a junior student in the council from her unusual behaviors.

Having heard about a suspicious meeting at night, Mikako starts investigating about it. Upon catching the sex orgy party in the school, however, she is forced to be involved in it.

Even after violated in the party, Mikako keeps attending the class with her strong mind. But she can no longer retrieve her normal school life again, because students
and even teachers repeatedly r*pe her with drugs to which she succumbed after getting too strong pleasure of it...

[From DLsite]