From MangaDex:
Three years have passed since the bombing of a classroom by a boy who rarely came to school. Now, the seven survivors of that incident have gathered in that classroom. One of the survivors, Takumi Shindou, informed the other survivors that one of them was responsible for giving the bomb to the culprit. With those words, the search for the accomplice began.

At the same time, a female student went to a private high school in Tokyo. "All of you will die with me", she said, with a bomb wrapped tightly around her body... Thus continued a chain of terrorism by bullied students, striking fear into classrooms all across the nation!