a disgusting boy

so, the girl i like just told me that there's a weird boy in our limits and derivatives class that keeps bothering her, he is disgusting, yeah maybe he is objecly handsome, but that doesnt give him the ability to harass woman, one of my friend (she has a boyfriend) had a problem with this man, he keep following her everywhere even if we were around, is so disgusting, he used to look under her skirt, but then we told our teacher and she told this boy's teacher, and it ended there, no punishment, not even a apology, and now he is being weird again but with the girl i like, and it's frustraiting 'cause she doesnt want to talk to the teachers, because we both know that they aren't going to do nothing about it, i want to talk to him and tell him to stop bothering girl (he is in senior year) not even that, just stop harrasing girls that are younger than him, what should we do? i want to protect her by any cost