Adult PC Game: Ingoku Jogakuen -Youmuin no Hiretsu na Wana- by UnderMoon Archive

This girls' school is the same as the others ... only during the day. At night, it becomes a place of lust and debauchery, led by a janitor named Kengo Gomi. A young guy who is not noticed during the day and at night begs for pleasure.
Three attractive girlfriends appeared at the school this spring. Kengo cannot ignore them, and the new students are already aware of the rumors about “special night activities”. The girls decide to sneak on them for the sake of curiosity, but stumble upon ... Kengo.