There are 15 students, all of them who accel at something. Whether it be a swimmer or a wrestler, they are top-class for their age. The main protagonist is a student with "Super High-school level: Luck", which, as you could guess, was accepted into the academy based on pure luck.
The academy isn't what you think it is, however. It was an apparent trap to all the students. Everyone inside is trapped inside with no possible way out. Then the antagonist appears.
He calls himself monobear. He has also claimed himself as the current school's headmaster. After his short briefing, he explains to everyone that they are indeed trapped. However, he can supply each student with food, drinks and everything to keep them alive. They are free to do as they please as well as having the honors of having a well-furnished room to themselves.
He then explains they can do that or they can graduate.
How do you graduate, you ask? By successfully getting away with and killing another fellow student. Only then, will you be allowed to leave the building.
And as you can guess, after a murder takes place, both an investigation then a trial follows right after.