The schools systems I have experience with do have gifted/normal/remdial tracks, and then various levels of special ed services. I agree that it's not enough. And especially so because there was never anything "remedial" about the remedial track. It was basically synonymous for permanently behind.
I taught math in Baltimore, and on the pretest I gave my students at the beginning of 9th grade, my students tested anywhere from 2nd grade to 9th grade in terms of current math skills, all bundled together in one room. On the low end of that scale, many students had been socially promoted to keep them with their age group.
What really gets me is that across the city, there are plenty of students who get delayed a year in their education, for a variety of reasons. Now we should be attacking those underlying issues, but that aside, it seems to me like we could much more proactive about getting those students truly focused remedial services, to either get them back on track or to at least maintain parallel pacing with their peers so that they get to the same endpoint, even if it's a year later.
We're rather fixated as a nation on the K-12 concept, rigidly linked with age, and with all subjects progressing in lockstep. I think we'd have much better outcomes with a more flexible system that recognize that a person's academic development doesn't scale so cleanly. I think even in a more flexible framework, we've got more than enough students to still take advantage of the economies of scale of systematic education.