How to stop this girl bitching about me?

[Lily]

Basically, this girl who is in my group of friends at school is being a right bitch. She is always making sly remarks about me, telling my other friends to ignore me, bitching about me to other people, putting up horrible statuses about me on FaceBook and just putting me down in general. I'm quite big (I've heard she was bitching about that) and I have really low self estem, this is all really affecting me socially and accademically as she is in some of my classes and I can hear some of what she is saying about me so don't concentrate as well as I had done before.
I'm fed up with her to be honest and I want it to stop. I want to confront her but she is quite a violent person and I don't want to start a fight but I will if I have to. I haven't done anything to upset her that I know of but she said she was hurt that I ignored her over the half term but I explained to her I had no credit because my contract had run out and my parents were in the middle of sorting me out a new one and I also had no signal where I was on holiday. I just really want some help because its really getting to me and she has already turned one of my close friends against me. Thank you for your help, it means a lot to me. x