Bratty problems?

There is a girl in all my cles. Pretend her name is Stacey. Stacey is in ALL my cles. We used to be really good friends, but lately she has been dissing me, ditching me, snapping at me and just being a brat. I am a pretty patient person but this has gone too far! I need major help now! Stacey is trying to turn the whole school against me with lies! Some people actually beleieve her rumors. She is just mean! She even lies to teachers and gets away with it! She spent 45 minutes in the bathroom once gossiping! I can not believe her! I know that the bathroom and lying things are not my problems, but the others are. Please help me! Before EVERYTHING gets out of control!