There are unwritten ones, at least.

As I said in my other post, in my school you were taken out of regularly classes, but still had to learn those things and do that work. That means the student has to agree to work 20% harder, usually at home, where they may not have that luxury. There are probably other considerations that aren't coming to mind at the moment, too, that also affect the number of children in Gifted programs.

Apparently race given the assertion of similar test scores

In my experience you are nominated to take an IQ-ish test and let in based on that score. I assume they mean test scores on regular coursework?