What should I do??????/??? I really want to kill this girl (not really, of course)?

You sound like you are a much nicer person. Please ignore her . Be pleasant if she speaks to you. but really you are better to keep to yourself. I know it's hard but when you are older and out in the work force Im sure she will not do as well as you. If you react to her she sounds nasty enough to want to get the better of you so it wont work .