This is an anthropology with multiple stories about some mature, Twisted theme.
The first story is about a guy who got called to a warehouse one day. There was a mask guy waiting for him and that guy dragged MC to the warehouse and raped him. This happened for quite some time. MC guest dad the rapist must be one of the three classmates because of their body size and height. But MC was fond of the guy sitting next to him and wouldn't mind is he was the rapist. At the end of the story though it turns out that there were more than one rapist. It got continued in the last story of this anthology but, spoiler alert, the rapist his teacher and two other guys that he didn't think was the rapist and the saddest thing is the one he is fond of is not even one of them.
Fukumen Danshi