So I'm trying to look for an old manga and it's bothering me super badly that I can't find it.

It's a super gorey manga about a student who transfers to a highschool where murder is one of the classes and everyone starts killing each other on day one. There's something about supernatural powers and it's related to the size of a mark on the person's body. I also remembered that during one of the swimming lessons the teacher released a shark into the swimming pool