We had some of the same issues/comments. Despite DDs grades they would continue to point out the errors, same with the other girl. We just pointed out that if she were not accelerated in math (ie she was actually a XX grader-fill in the accelerated grade) they would not suggest moving her back because she wasn't getting a 100 or even an 88. If they thought mastery skills were lacking, they would pull those kids and teach them in small groups (or whatever method). They wouldn't normally move a kid who was getting As (or even Bs) back a grade so why did they think it was OK to move my accelerated child back for getting the same grades? When they suggested undoing the acceleration due to these errors, we also questioned their trust in the teachers ability to grade the students (ie if she's getting a XX, do you really think she's not ready to move on? don't you trust the teacher/curriculum?) All kids have gaps, not just accelerated kids.
You might also want to point out the self-image confidence issues of not continuing the acceleration esp. given that grades are good and I didn't hear you say anything about social issues.
FWIW - adults make computation/sign errors all the time as well. It's why those of us in STEM fields love excel and write programs to do the computations and why professors usually give very liberal partial credit and/or grade on a curve.
