Guy from karate cl giving trouble at school?

Ok, this guy from my karate cl (2 belts below and one year younger, but ALOT bigger, im only 5"0 lol) He keeps cursing at me like going in to hit me ast school if i walk by him. It hasnt been bothering me, ive been ignoring it, but today he went to trip my little sister, (3 years younger) so i yelled at him to stop it or i'll make him sorry, and he said oh yeah? come on then. Why is he doing this and what should I do?