... more bizarre still, children are shamed/penalized at school in K-2 for being "off task" when doing things that aren't in the scope of the curriculum, for being divergent thinkers, for being unwilling to stay within the "lines" as it were, and then suddenly in 3rd grade, they are tested to see which of them has managed to resist this kind of pervasive operant conditioning.
Those who are most resistant to operant conditioning, we'll identify at that point. The rest of them have apparently "evened out" by then.

Maybe "evened out" is actually code for "given up" or "become compliant."
I have to wonder just what that third grade selection process is
actually selecting for-- because most HG+ kids simply can't
take being that compliant with things that far from their proximal zones for years on end. Not without figuring out some coping on their own-- and most of that coping is going to be about on par with what one might expect 4-8yo children to come up with. Defiance, refusal, daydreaming, clowning, etc. all offer some rewards in a pretty unrewarding environment, after all. Sure, as an adult I see how those things are counterproductive or even maladaptive in the long run-- but for a 6yo, they offer immediate rewards, not the least of which is an alternative to doing mind-numbing things like an automaton.