We've run into similar problems at our MAP-testing schools, although the schools do not threaten to remove students from the programs if they are underchallenged...my DC are just sometimes underchallenged and the school does not always have a program for them.
I see your dilemma and you are not alone. I have two DC who often have extreme scores and the school has usually not volunteered to teach them at the "level" their own testing suggests. The teachers usually just remark, "Wow, those are high." Our schools do use differentiation, though - it just typically has not been enough, as they are stuck in their correct age-level grade. The scores usually have just placed them in the high grouping for grade.
Our solution has been to talk to the schools and teachers, while afterschooling at the correct level at home. It is a VERY imperfect solution, and honestly, is not fair to my kiddos. The good news is, though - my DC are making progress.
I wouldn't refuse to test - I see the test as one objective measure of what my DC was able to learn...and incidentally, DD9's MAP scores correlated pretty well with her EXPLORE scores this past year, as well as what I see IRL. So I do not think the scores are meaningless.
Our teachers are trying, but our sticking point is getting differentiation beyond grade level. DS6 is at the 99th percentile in math - 3 grades up. DD9 is at the 99th percentile one grade up in reading and math, and has very high percentiles - for 8th grade. Her scores best correlate with the average G&T 8th graders in our schools (she is "off the charts" for 8th graders in the "regular" classes), but she is in G&T 4th grade. No grade skips here, although DD is now old enough to be in G&T - DS is not. Our work with the schools is ongoing, but at the end of the day my goal is for my children to learn. I will help them if I have to.
