Well, anxiety is the obvious first issue since test scores are the biggest factor in her grade decline. After that, I think working memory, processing speed, and executive functioning (in this specific context) could be contributing factors. It has been my observation that most kids of average or above intelligence are actually ready at a conceptual level for more accelerated math than their school provides to them; however, most kids do not have the working memory, processing speed, or executive function to handle the accelerated curriculum as designed. Schools hold half the kids back because it is much easier to teach them when they are not just conceptually ready but have all the supporting tools to succeed without accommodations.
DS12 is one year behind your DD math-wise and hasn't hit his wall yet in school math; however, I have seen this phenomenon in competition level math with a time element like the AMC 8 and AMC 10 over the years. He simply made far fewer "careless" errors after he mastered/internalized concepts versus when he just understands these concepts - this conforms with what I have heard from some top math competitors.
