Masked Reunion (Kamen Dosokai)

Other name: Kamen Dosokai

Synopsis: Shintani Yosuke comes back to his hometown where he lived until graduating from high school. He attends to a school reunion where he meets his three friends. They decide to kidnap their ex-teacher who used to punish the students with physical force. They beat him up and leave him there. Shintani and his three friends thought it was a joke, but the teacher is found dead the next day.