Responses
1. The original body the MC is in, was bullied by those students. So to ensure they would stop thinking him as a weak link to of course the MC has to deal with them.
2. The teacher is his reincarnated military comrade.
3. He go involved with the gangs because the bullies called the minor gang members to attack him. And because one of theirs got defeated by a newbie of course the whole gang got involved.
It’s literally all connected, you might just need to reread
I honestly want him to go back to military and make the place upside down. Why is he getting involve with bullies, teachers and gangsters?