I was in one of these programs and here's how it worked. We all went into the cafeteria around 3rd grade and took a test like the SAT. Then we were evaluated individually in areas of math and pattern recognition. Those who scored the highest got out of Reading every week and instead went to an enrichment class where we learned miscellaneous "enriching" things like Shakespeare, architecture, programming, history of art etc.
The problem with this model is twofold:
1. First it ostracizes the ones in that enrichment class, and demotivates children left behind. It also causes rifts and resentment among friend groups. Every day the other children see Sam and Sally leave class and go to a "special kid room", and this turns into jealousy and bullying. The kids in the enrichment class feel bad for being there, and the kids not in the enrichment class feel bad for not being there. We learned a whole lot, but I think this model could have been implemented such that we could still have learned those things without the associated social costs.
2. It's not a great selection mechanism. For example, they passed over some of my friends who, in my experience, are way smarter than me. Why? Well one had slight dyslexia, but he was a math genius. Another is a music and art genius, but she wasn't great at numbers. So we have people who would have benefitted from these programs, arguably both have a very high IQ, and yet this whole process left them feeling "less than". They had to sit in boring, sad reading class while us "enrichment" kids were putting on a production of Macbeth. If that's the kind of thing we were doing, why wasn't my artist friend invited? Because she couldn't do algebra in 3rd grade?
So if there's so much social pressure, resentment, bullying, and the selection method isn't even that great anyway.... then what are these programs for? Can't we come up with a better model that enriches all kids? Instead of asking some magic sorting hat and trusting it's selecting the best students?
Anyway, that was my experience decades ago... maybe things are different now. Obviously I have no idea what's going on with 3rd grade social dynamics these days.