I am not sure what everyone is talking about schools not being tracked already. At least when I went to high school in California there was at least 3 levels, regular classes, honors classes and advanced placement classes. Elementary school had the gate program. I was in the advanced classes all the way through, but there were a few classes that you had to take like Drivers Ed (I know that dates me), Physical Ed and Health, that were mixed. It was actually a rather disturbing experience. It was like being put in a room with wild monkeys. It did give me an appreciation of how the other half lived though.
So I think a tracked system is valuable as it lets people learn at there own pace. If we really wanted to improve our system though we would tailor education not only to level but also learning style. For example I can learn very quickly from a book or doing something, but am awful at auditory learning. I usually read the chapter and then slept through the rest of lecture classes. In college I just skipped the lecture classes and showed up for the tests. By focusing on how each student learns and catering to that they can learn far more quickly regardless of their general intelligence. That is one of the reasons why homeschooling is so much more effective.