I am missing any mention about the child's actual academic needs, as opposed to one parents determination that the child must be eligible for the accelerated math track and the others parents disapproval of that. For all we know, the child may love 6th grade maths, and be miserable being stunted in the classroom - or be hothoused to death and be miserable with her nose kept to the grindstone.
The issue fundamentally is that child's academic need is very much a judgement call. Having fun in lower grades has worked perfectly well for a lot of kids but has been a disaster for others. Also measures such as NWEA MAP is a very blunt tool. So impossible to base any judgement calls based on it.
My feedback here would be to work with the school administration or to an alternate school. There is absolutely no value in fighting with them.
