Guy in school calling me ugly and being really mean!!! HELP!!?

ok so there is this guy in school and him and his friends are always being mean to me! they think i love my self, they say "oh u think ur so beautiful when your are really an ugly man!" and call me ugly all the time!! but i dont love my self and i dont tink i'm beautiful i actually think i'm ugly! and he just makes me feel even more ugly by saying all that he does!..why does he say it? why is he SO mean to me? plz answer x