We had the gaps issue when DS was in the local school (now he's at a GT magnet). They would tell us that yes they did pretest him but he didn't do well enough to be excused from the unit. But it turned out many times that it was just that DS didn't remember the terminology, and he actually did know how to do the work. After much rigamorale, we came to a compromise that DS would only have to go through the parts of a unit that he didn't show any mastery of (85%, I believe). If there were areas he missed, the GT teacher would do a very quick review to see if he really knew it, but just didn't remember terminology or just needed a quick reminder. Then the GT teacher was to start DS with new math concepts. If it turned out he hadn't ever learned a particular unit, he would join the regular classroom for that portion. Also, if the teacher found that he learned any concept very quickly, DS would be able to go to the GT teacher. This was the plan, but we never saw how well it worked because an opening came up mid-year at the GT school, and we transferred. It may have worked, but then you would need to have a GT teacher or other staff person around to basically do enrichment or tutoring. The easier thing would be to send the kid to the class working at the kid's level, but our local school would never agree to that, citing poor handwriting.
Don't know if any of my ramblings will help, but I wish you good luck!
