pinkpanther - I'd have to say that private is more likely to be accurate, but it depends on the child and the reason for the testing. If it's merely identification for a GT program that you're looking for, and if the school testing yields results that would qualify your child, then it would seem that's all that's needed. If, however, you have anything like twice exceptional concerns, or the school testing does not yield results you can trust due to the testing conditions, or your child may be particularly difficult to test for some reason, then private testing would probably be far more accurate. For some kids, like mine, there's no substitute for one-on-one testing with a professional who understands gifted kids.
We did private testing for DD6, 2E, at the Gifted Development Center, and I was blown away by the amount of info that came out of that (it was the WISC IV). They pointed us in the direction of other things that have helped greatly with her learning issues (OT, vision therapy, etc.). The experience was invaluable. I am able to apply a lot of that knowledge to my other kids, though I'll still want to do private testing for them as well, if and when I decide the time is right. In DD's case, the school was useless - she floated along in K, performing average and not seeming to learn anything, and no one connected with the school had a clue. I decided to do the private testing on a hunch (well, several hunches). This led to a school change. We'll probably repeat the private testing for dd if we need to apply to a gifted program, since they said her score is likely to go up once certain issues (especially vision) are addressed.
In contrast, at the moment we are going thru the IEP process for DS5 (for OT for fine motor/handwriting and speech) and I assume he is 2E as well (more extreme in every way compared to dd). This testing is still part of their early childhood program since he's not in K until next fall (my purpose is to make sure he gets services at the very start of K). So, in their defense, this school team was not assessing giftedness, nor did they appear to have any special experience with gifted kids. The school psychologist was brought in, and performed a cognitive assessment called the DAS II, which is not an actual IQ test as much as a way to diagnose learning issues (the results were not surprising, though we still don't know the full extent of his knowledge because he is not interested in sharing/performing/talking - long story - very introverted). The psychologist mentioned several times that the scores would have been lower in the high areas if she had followed the scoring rules more closely in that she had given him incentives (stickers) to answer questions, which apparently is contrary to scoring rules. (Um, are we looking for a truer measure of his abilities or not?) Anyway, the tester from the Gifted Development Center was very accustomed to dealing with introverted kids, and in the beginning my dd was answering through a stuffed bunny. Big contrast in that regard - the private tester was not only very experienced, but very motivated, to get as accurate a picture as possible of my dd. This is the basic reason, my kids' extreme introversion (dd was supposedly one of the most introverted they had ever tested - how is that possible? and yet her brother is more extreme in that regard), that if and when I need a real IQ score to apply to a gifted school/program, I'll definitely go for private testing for my kids.
Somewhere I read (on the GDC website?) that the highest score a person had achieved was the closest to their true ability level, since one cannot achieve greater than their ability on an IQ test. So, in the case of all the different tests your kids have had, I'd assume the highest ones were the best, among the existing group of scores, in terms of accuracy.
that's my two cents, though I'm not sure if it helps
