Captain Underpants

This is a series that talks about this hero. That is always on underpants, because he is initially a principal, but this two students always get in trouble so they get this hate for him. so one day this two students order an "hypnotizing" ring and they hypnotize the principal to become this super hero, and it becomes this series where he fights imaginary enemies like toilets, and this has become a series of 12 (I believe) books.