Maybe the idea is that limited teaching is better done to people who are more likely to be able to make usage of that knowledge in life?

That is the idea in theory. But in practice, apparently North Carolina teachers, under pressure from rich parents, don't want to teach algebra to talented kids from poor and black families - i.e. don't want to teach precisely those who would be most likely to use and benefit from that knowledge.

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Why would rich parents be against this? Also, why are rich kids in same schools as poor ones? That would be unimaginable in "socialist, equitable" EU and makes no sense to me overall.

Because Racism