How to get this girl off of me- I dont like her....?

This girl that just happens to sit behind me for the first 3 hours of that day seems to like me and its really bothering me because she makes it so obvious and it I and everyone in my cl knows shes hitting on me and I dont like her at all. Personally I think shes ugly and really annoying, and have thought this since before this last week, and now I kinda wanna just die when shes around me. I dont want to act pissy or 100% ignore her because I am friends with her friends and most of them are cool but I dont like her on me all the time