I was lucky, was put into a class that corresponded to about the top 2% of a suburban population, probably about as filtering as the 30 or so best colleges. Put in that class starting in 3rd grade. It was such that I actually did OK in 3rd, had a VERY hard time in 4th grade (too hard, too much work), but emerged in 5th grade as in the top 25% or so of this very filtered class.

Many of my friends homeschool, more at younger ages than older. Often high schools have enough stratification in classes offered that a smart kid can merge back in taking APs and so on.

Resources for home schooling have never been better than they are now.