That selectivity thing has more to do with how we fund and operate schools and less with the requirements of education.

If you look at NYC, they have a model of neighborhood elementary schools and location based high schools, but also a network of merit based advanced and magnet schools.

When I was in high school in a rural upstate NY town, we had tracked programs with "smart" and "not so much" kids and rarely interacted directly with kids more than one track up or down in school. (There was only 90 kids in the graduating class)

It was a real eye opener when taking some mandatory class as a senior (something like civics) when I discovered that my writing partner didn't really know how to read or write. To me that's the tragedy of schools -- that should not happen.

High school is both mandatory and selective. Most schools have multiple tracks: vocational, general, college prep, honors / ap. Also private and some public high schools, like specialized magnet schools, can be more selective in admissions than many universities.

In small towns the tracks may end up being a couple of different english and math classes, band vs autoshop (if either is funded) and how much chemistry/physics you complete.

Just let people stop going to high school at age 16 and make the last 2 years more rigorous.

There need to be other changes to go with that, but the people electing to stop school would be happier and I think wouldn't miss out on much learning.