I am really worried there is something mentally wrong with a kid from my school. Could he turn violent?

Talk w authority. If you are concerned about someone's mental health, you should express your concerns with your parent(s). They'll be able to advise u what to do about it. If you fear the kid may become violent, it's absolutely necessary that you talk with your parents and a person in authority (teacher, principle, guidance counselor..) at the school. Mental illness is a very complex thing; go with your gut.