I attended a public school in a Minneapolis / St. Paul suburb. Nothing particularly special about the school that I can recall today, although it was in a school district that included grad student housing for University of Minnesota so maybe the student body, as a whole, was more motivated than you may have found elsewhere.

Classes were tiered into ability, even at this age (I do not remember what the rubric was for deciding your tier) and for math classes I was pulled in to a self-study course along with 2 or 3 other students. There was no 'formal' class and instead the teacher would spend varying amounts of time with each student as they worked through the material on their own. Each student would work at his or her own pace.

I think this sort of system would do wonders for those both at the very top and at the very bottom. In college, I found office hours were somewhat similar in that students both behind and in front could get focused time with a professor to either catch up or advance even further ahead.

This is ignoring any budget and teacher constraints, but this seems like a real issue for both the 'top' and 'bottom' students.