...it's hard to see how IQ could be too drastically overestimated.
I'm not saying that this is the case with the OP's child, but I do believe that IQ can come in "too high," so to speak and have seen the numbers drift down over time like polarbear mentions in some instances. The highest number ever achieved isn't necessarily the most accurate number forever.
In the instance of early numbers being higher than they later wind up, I've seen it happen in instances where the child is at least above average and in a very enriched environment. For instance, getting even one question wrong when you are at the upper age limit for a test will significantly diminish your scores (the ceilings we all hear about). The same is true on the other end: younger kids who get even a few "hard" questions right will wind up with their scores shooting way up. A kid who lives in an environment where s/he is exposed to a rich vocabulary, has opportunities to acquire knowledge (books are read to the child, science museums are visited, etc.), etc. is a child who may wind up with a higher score early on b/c his vocabulary and knowledge in relation to his age peers in the norming group may be better. This may or may not be due to factors that will impact the child's general intelligence in relation to peers as s/he and those peers continue to develop and age.
In regard to the OP's child, though, the best advice we got was to do what is right for your child *right now*. If right now, the school system is going to make your child unhappy or create an environment where she won't learn anything, doing something different is totally reasonable regardless of what her long term IQ scores may or may not be.
