Kamen Dosokai

Shintani Yosuke left his hometown right after high school graduation. He wanted to leave behind memories of his older brother who died in an accident and a teacher who was bad to the students.
Now, Shintani Yosuke is back in his hometown due to a personnel transfer at his work. He decides to attend a school reunion and meets 3 friends from his high school days. Yosuke and the 3 friends kidnap their ex-teacher who punished the students with physical force. They beat him up and leave him there. Shintani Yosuke and his 3 friends thought it was a joke, but the teacher is found dead the next day.
(Source: AsianWiki)