Whats this girls problem?

There's this girl at school, and I've gone to school with her since middle school. She moved here from upstate, and everyone made fun of her because of her accent, said her face looked like it had been hit with a frying pan, etc. We became friends. Three years later, we've both gone our separate ways. Needless to say, I took the high road, she went down the wrong road. She's slept around with numerous guys, does drugs, drinks to excessive amounts, and cares nothing about her grades. I'm not perfect, but I don't need drugs or alcohol to have fun. I care deeply about my grades, because I want to make something of myself and go off to a university. I have done nothing to this girl. She won't say anything to my face, but when she's out of my reach she'll say things like "Ew, Nasty!" "EW, EW, EW".."Ew, I almost died from looking at her", loud enough to wear I can hear. I decided the best thing to do would be to ignore her, since I don't want to get suspended from school etc. I cheer at my high school, and getting into a fight would have me thrown off. Last week she decides to run into me on purpose. I knew it was intentional, since she ran into me so hard her purse fell off her shoulder. I called her a *****, and walked on. The only thing I can think of that may cause her to act like this would be the guy I'm dating..We've been dating for a month, about the time she started acting like this towards me. Back in sixth grade, she was "talking" to him, but then she eventually stopped talking to him. She thought they were going to date, but then she chose to do drugs and sleep around with everybody. This guy is a decent guy, who does nothing of the sort. I think this could be why she's acting like this, but idk..What do you think? I'm so tired of this immaturity. Friday, the principal called me into his office and said he's seen this going on, and said he was on my side. He asked if he could help me with this problem, or if I could handle it on my own, I said I wasn't sure as of then, and he told me to think it over over the weekend. He said he would talk to her and find out what her problem was, but I don't want her to think I'm a tattle tail, etc. And I know she would try to fight me after that.. But what should I do? Is she just jealous?