Someone is causing trouble for my younger sister.

Salaams, I really need some advice. My little sister got really upset recently and confided in me that a guy from her school has been harassing and picking on her for almost a year now. One time, he even grabbed her arm so hard it really hurt. My heart just breaks hearing this. She made me promise not to get involved, saying this person has a bad reputation and she's worried I might get hurt. But I feel so helpless and like a coward for not being able to protect her. What should I do? I'm just lost.