I completely empathize with the OP. My eldest, while in the range mentioned by OHGrandma on the one IQ test she took, really just does not have peers locally. I do think that she is somewhat more HG than the IQ test indicated. However, I don't think that she is PG/DYS level. She still seems radically different from most of the other gifted kids she's paired with in school and doesn't have many peers in extracurricular activities when the kids are the same age as she.
Things have been somewhat better with her grade peers being btwn 1-2.5 years older since she skipped a grade. She is much happier socially.
Back to your original question, it seems like you have two options:
* let parents self select while clearly stating that it is for highly to exceptionally gifted children and hope that it doesn't draw a bunch of high achievers and mildly gifted kids whose parents think that they are highly gifted; or
* do something like Intertel vs. Mensa -- 99th percentile vs. 98th -- set the qualification at the 99th percentile (on a composite achievement test, IQ test, or whatever you deem appropriate).
