I'm with Ultramarina, I've run into tons of anti-tracking research and advocacy. There's no denying its out there. Most recently I was reading Jo Boaler's "What's Math got to do with it" which has a whole chapter devoted to the subject.
More charitably speaking, the main motivation seems to be that lower tracks have very poor results in raising the performance of kids that are enrolled in them and that school systems disproportionately channel minority kids into the lower tracks. Both of these facts are fairly non-controversial. There also have been studies showing that in tracked classes the actual ability level of students doesn't differ between the tracks due to parental pressure to have kids in higher tracks and the above mentioned implicit biases in the system. I can also believe this to a certain extent does occur.
Then you hit the crux of the dispute which is whether higher achieving students are harmed by being untracked. The contention is that this is not the case. But this is where I part company with this line of reasoning because it fundamentally assumes most students are the same or close enough and can handle the same level of rigor so there really isn't a compelling need for acceleration/tracking. I'll leave it at that but suffice it to say this is a thorny problem.
Here's some more reading if you want to see another sample:
http://www.ascd.org/publications/bo...-Is-and-How-to-Start-Dismantling-It.aspx
