> All of this seems to suggest that there is something about the American education system - and perhaps our overal culture - that is failing our children, and our black children in particular.

You're missing a huge cause before jumping to this conclusion. African immigrants don't just differ from African-Americans in that they weren't born here; they're also a subpopulation that's gone through the dramatic filter of the US's legal immigration policy. If you have a filter that is explicitly trying to select for the educated and economically productive, it's practically a tautology to say that the outcome of that filter will consist of a more educated and productive population than your control group.

I can't imagine the same dynamic wouldn't hold for recent immigrants from most regions vs their many-generations-American counterparts.