Tada, Soredake de Yokattan desu

A psychological mystery about middle school bullying and depression. The sister of a suicide victim investigates a school to find out the reason why he took his own life. A single boy was accused of extorting an entire group of students and driving a boy to suicide, but who was the real bully? Grand Prize winner of the 22nd Dengeki Novel Prize.
(Source: Anime News Network)