My friend is using me, how do I cut off our friendship?

If you really feel that she was using you, and you have nothing else in common and your really don't want to be her friend let the relationship just die. Don't go by her locker, when you see her in the hall let your "hi" turn to a nod or raised eyebrows, eventually it would just turn to a quick glance and then you will eventually pass like strangers. Do not pick up her phone calls, or return her emails. Once a friendship is dead, let it be. THe following year you may find yourself in her class again, just don't sit near her. Before you do any of this though, make sure you really want it.