Wanted to add that the bad rap of tracking IMO results from times and or places (Germany is one to this day) that put all the low ability kids in a separate school which then ends up concentrating all kids from risk situations, from low SES backgrounds, with parents living in poverty or being unemployed or uneducated, and second language learner and kids with mild learning disabilities and with major behavioral problems, every one else trying to get the hell out of those schools and make it into the medium ability schools. I think those kinds of schools concentrating a high risk population shouldn't exist, and they have been proven to make every kid even worse of than the would be, the concentration of risk factors obliterating any benefit the targeted instruction might bring.
I am not sure about separate remedial classrooms either - makes it too easy for every one, teachers and kids alike, to just give up. Combining low and medium ability classrooms, instruction targeted at the middle, with lots of tutorial support for those who need it, push in, pull out, after school, whatever, might be best. It's sort of what Finland does, where the level of instruction is high enough that 50% of all kids will at some time have been in special ed. But they do not separate their high achievers and do admit that they often lack challenge.
