I would say you have a right to say that either that girl gets moved to another classroom or your daughter does. That would be the first thing I would do at this point. It sounds like the teacher has been trying, the administration even sat down with the other parent, etc. This needs to end, and it may just be that physically separating the two is the only way. If this is a private school and there is only one class, I'd say they have cause enough to expel the other girl as a danger to other children.

And yes, this very well may be why your daughter is acting out.