The more specific you get, the more relevant the "population" becomes.
I'm sorry, but this doesn't make any sense. What does this mean?...they teach students tracked by ability level.
You're just trying to make it complicated. Statistical analysis has a number of ways to correct for outliers and disparities between demographics. It's a simple matter of holding things constant and observing averages. Math is not nearly as limited as you make it out to be.And ability levels are a) subjective, and b) not distributed evenly among the distribution curve. There isn't one g that will account for abilities in all areas. And those "ability" levels are often significantly influenced by socioeconomic background, family culture regarding learning, etc. And some teachers are better with some sorts of students than with others.
I'm just saying its more complicated than you may believe.