Well its rather simple, a school has an extra pupil in class 7, who is dead. The fun part is they don't realize they are dead, and thus are inflicting a curse upon the whole class, killing them off one by one every month or whenever information on stopping the curse is leaked from mouth to mouth. In a sense, it's an irreversible curse that nobody even knows about, and the only way it will lift is if that dead person dies again, however it will restart itself the next year.
I must say I think I'm a genius for this one.
I must say I think I'm a genius for this one.