Masked Reunion (Kamen Dosokai) (2019)

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.