It seems to me that the main problem with tracking is this:
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rather permanent group consisting of age-peers moving together through the grade levels. Pupils are generally advanced in all subjects. Commonly receiving curriculum instruction one grade level ahead of gen-ed age-peers...When kids get on the "track" they typically do not leave; Similarly, new kids may have a difficult time getting on the track, as a "track" is generally considered closed...a student is either in it, or not.

So the problem is that it's permanent. If it wasn't, kids who underperformed/slacked off/etc and improved would be able to move up, and kids who started in it and didn't deserve it/work hard could be removed. This also creates motivation to try to improve. There's mobility. Furthermore, students could choose advanced classes in, say, math, but not in English.
However, if the classes were a year ahead, versus just being more in depth and challenging, that would cause some problems.

...I'm confused. Maybe I'm misunderstanding, but that's what our HS's honors classes look like. Sure, they're not representative of the student body; sure, they're lacking for HG kids. But still -- kids can pick and choose honors classes, move in and out, etc. as long as the teachers/parents approve.

Or are they keeping a similar system and eliminating tracking for, like, elementary / MS?