What will happen inside the gifted program? Will it be a more rigorous approach to subjects, with somewhat more enrichment than the normal course, or an accelerated program, or a focus on creative thinking? What percentage of students do you want to qualify for it?
Basically, your criteria for entrance to the program should be determined by what you do within the program. And really, both sets of students likely need some form of specialized education, so it isn't necessarily right to exclude either.
My kids' school system uses more of a focus on iq (and iq like measurements like the cogat). No idea what life within them is like yet because it really doesn't start until 4th grade (oldest is currently in K).
