Can't Stop Cursing You Vol. 1 (manga)

It was just an ordinary day at Yami High..until it wasn't. A ringing cell phone and the splatter of blood marked the beginning of a deadly game. The rules: Contractor Mikiya has thirty days to curse-kill thirty people-or meet a fate worse than death. On the other side, Curse-Breaker Saeyama must identify the killer... without falling prey to one of Mikiya's curse-kills. Only time will tell who will emerge victorious...