Maybe going too far off center from the thread... but I've been thinking about this on and off for years. This and having a kid sort of brings it together more. I remember in the eighties reading an article in I think Omni about a "mastery based" approach to education. Where once you achieved 90% you moved on to another topic (and I was like "holy $#, wish school had been like that.")
If you pair that with a measure like "repetitions to mastery," then you get a "you got it, move on" acceleration along with a sense of how much time/practice each student needs. Obviously the mastery list is dynamic, similarly the average repetitions to master change as students may change their rates. If you look at the sort of concepts/topics common core points at, you don't necessarily have to ask for qualitatively different instruction unless it fits a concept that has been mastered. If you've mastered "extracting theme" then your reading instruction should include discussions of theme or whatnot.
Fairly high level and unpolished, but sorta serves as my mental yardstick, and I think done right it dodges tracking and its conceptual baggage.
