Rather than try to put the girl only in a higher level math, have the parents tried to get her in a gifted and talented program? My son went from elementary thru high school in a G&T program and did very well. Most such programs will require that you do well over all (as opposed to one area such as math), but usually that’s not a problem.
It’s been a while, but I think that the G&T call is not one the classroom teacher makes, but is made thru the school district after testing. You might try that as an end run around the teacher.
Your options depend on where you live. G&T program might be the most straightforward solution, but not all school districts have these. My son did G&T starting in 5th grade, but then we hired a college student to tutor him once he got beyond the level of math they covered in G&T. He had run through all of H.S. math by the time he was H.S. freshman, but fortunately we lived in a very enlightened public school system which let him start taking college-level courses in a local public university while he still was in H.S. It was a hassle logistically, and we sometimes ended up relying on college student drivers, but well worth it.
Because we were grateful to get the special permissions needed for him to come and go from H.S. on a non-traditional schedule, we never pushed on the issue of whether the school system should be defraying any of these costs we incurred: had he been developmentally impaired, the cost to “mainstream” him presumably would have been much greater and I further presume the school district would have been legally required to foot the bill. But my general impression is that most school districts feel far less obligation to meet the needs of G&T students. Good luck.