[BobaFloutist]

>I am not convinced that separating students by ability harms anybody's academic performance.

You're not convinced that it can do that, that it typically does as currently implemented, or that it always will do that?

Presumably you agree that as long as a student is capable of keeping up with the material, they'll do better (in the long run if they get placed in) in a higher level class. Otherwise there would be no benefit in separating the students in the first place! I assume you also agree that we don't have any perfect tools for assessing a kid's potential. Thus, when separating kids, we will necessarily get some wrong, to their detriment. So, I hope, you're willing to concede that this could harm their academic performance.

Now, the extent to which that happens under any given paradigm is very much up for debate. But hopefully this illustrates how separating students by "ability" could harm "anyone's" academic performance.