In'youchuu ~Chronicles of Demon Exorcism at the Violation Academy Final by inkerbell - Adult Sex Game

The story starts at the office of the demon hunter company, the "Black Cat Brigade". A phone call informs them of a case regarding a haunted school, the case sounds simple and since Mikoto is afraid of ghosts and Yamato is too much of a trouble maker, Takeru decides to take care of the case by herself.
One week passes and Takeru has interrupted all communications, which is very unlike her, so Mikoto and Takeru decide to go looking for her at the school.
Once there they find a pretty normal situation but no trace of Takeru, so they decide to act as students, with permission from the principal, to look for her without raising suspicions.
They also meet with Sui, Yamato's ch*ldhood friend who clearly has feelings for him, who says she met Takeru right before she disappeared.
It quickly becomes clear that something strange is going on in this seemingly peaceful and respectful private institute in the middle of the mountains....