J.B.
Has she been tested for learning disabilities or AD/HD? Sometimes behavior problems are covering up feelings of inadequacy or a neurological inability to control her behavior.
If those have been ruled out, I would seek out an adolescent therapist. Either something is bothering her and she's acting out and gets some kind of reward from this, no matter how perverse, or she is very immature and has problems with impulse control. In either case, I think that a good counselor could help her out tremendously. Somehow, this behavior is satisfying a need that she has and can't articulate or probably even identify - insecurity, attention etc. - and in another year or two if she continues to act this way, it will damage her peer relationships. The disruptive kid is amusing at first but then quickly becomes irritating.