Comment by liveoneggs

I find these arguments ignore one of the most obvious and important parts of school: discipline/behavior.

A really small number of disruptive kids will destroy the learning of the entire middle. The top kids will figure it out at home and survive, or their parents will separate them through brute-force - a few borderline high achievers will probably be brought down too.

In the poorer neighborhoods around me the school performance is actually shameful and the kids are being subjected to the worst of their peers constantly, destroying their ability to succeed. Many will (hopefully) drop out/get arrested by late middle/early high school but the damage they do to their entire neighborhood along the way is massive.

Cohorting the highs and the well-behaved middle would probably work out just fine but unless you can eliminate the disruptive and the very-behind it's just the worst of all worlds.