Bummer, I'm sorry. I do find that many educators have no idea what IQ scores measure nor the difference btwn levels of giftedness and how rare HG-PG is in their population. I had a similar conversation with the GT coordinator at our neighborhood school re my dd11 when we were considering other options for her late in elementary. She assured me that she had many students like her and didn't seem to understand the difference btwn 99.9th composite IQ vs. 95th percentile in one area on the CogAT: the 95th in one area on the CogAT kids had very similar scores in her estimation.
The other thing we found was that, at least at our neighborhood school, there is a lot of retesting on group ability tests going on and a lot of prep for these tests. B/c the scores on these group tests change a lot when prepping occurs and b/c they also tend to jump around when kids are retested until they hit that magic 95th number, educators often feel that IQ tests, like the group tests, aren't measuring something innately different so much as the kid having been worked with like your principal seemed to infer.
Ultimately, we've found that with HG+ kid(s), it sometimes isn't a straight road. You may start somewhere and have to change schools later or something else. I try to follow the advice my dd13's GT teacher gave me when we were looking @ her skipping 5th grade: do what's best for her right now. Right then, the grade skip was the right thing and, fortunately, it has turned out to be good long-term as well.
Right now, if the best you're going to get is the classroom teacher differentiating, then you'll have to work with her to find ways to make the differentiation good enough. Later it might be something else. Later the principal might change, the GT program might change, you may wind up working directly with the district GT coordinator for accommodations, who knows.
I hope that this year goes well, though! Good luck.
