Gokushou

Eigo Ayata, the leader of the Ayata-gumi, has been a member of the yakuza for 40 years. He is infuriated when he learns that Makoto, the only person who always treated him kindly, has attempted suicide due to bullying.

He tries to go to her school to confront the bully, but he is kept away due to his appearance.

He decides to enter a machine that can make him young again, and joins the school as an elementary school student!

(Source: Young King, translated)