Maybe the issue to explore is why she got an 81% in the first instance. Was it failure to show all her work (a common issue with gifted kids who skip steps but arrive at the right answer), sloppy mistakes that had nothing to do with mastery of the material but rather lack of attention to detail, or a failure to complete all assignments. These are things that can be "worked" on at any level and repeating the same material would be of no benefit. I think your best chance is to get her former teacher to explain that she is ready to move on despite the grade, if in fact this is the case.