Nusumareta Houkago

Shirakawa Ayumi has transferred to a new school after her friend Masako recently jumped from the top of a building and killed herself. Things finally seem to be turning around for Ayumi. Her homeroom teacher is handsome, she's already begun making new friends on the first day, and she found out that her uncle even serves on the school board. But all of this suddenly changes when one fateful day, her new friends and she decide to pull a silly prank on one of their teachers, triggering a series of violent murders...

Based on the novel Shisha no Gakuensai, by Jirou Akagawa.