Ijime no Kejime

A black notebook arrived for Amano, a high school teacher. What was spelled out there was the story of fierce bullying that one of the students who Amano was in charge of received. After 17 years, Amano, who quit teaching, continued to search for the student who was a victim of bullying, but he has yet to find out where he is. At that time, his classmates at that time began to be killed one after another...!?

(Source: Kodansha, translated)