There's a girl at school, that makes me uncomfortable?

There's a girl at my school (I'm a freshman in high school), and ever since 6th grade, this girl has had a problem with me. She spreads rumors about me that I sleep around and that I'm a whore, etc. and I always hear her whispering awful things about me behind my back. She is usually snappy with me and disrespectful, and I often catch her staring at me. Could she be jealous/envious of me? Obviously she doesn't like me, however I don't see how you could have so much animosity and hatred toward someone that hasn't done anything to you. Honestly I think I look better than her, and people have even TOLD me that I look better than her, however I'm not sure if this is the reason why. The reason why this situation bothers me is because I have classes with this girl and sometimes I cannot focus on my work because of the discomfort and awkwardness that I feel when I'm around her. For some reason, I always get stuck either sitting next to this girl or in front of her or something. I've even TRANSFERED OUT OF CLASSES due to the discomfort that I feel. What should I do?