In my view, the key insight is that the educational system together with the labor market and other social institutions, both formal and informal, must provide a gainful and dignified path for people in all percentiles of intellectual ability. For this, two things are necessary: an educational system (and other supporting institutions, primarily functional families) that effectively direct people of all ability levels towards occupations that are realistically within their reach, and of course an economy offering gainful and dignified employment to people at all ability levels. Without either of that, what results is a large underclass with the most awful social pathologies rampant.

How is what is realistically within people's reach to be judged, and who does the judging? I've seen enough accounts by low SES people who were told they couldn't do things they actually ended up accomplishing.

On the one hand, people really do have intellectual limits, and on the other, status enforcement is a pervasive habit. It's very hard to tell whether you've got your tracking right, and keeping capable people away from what they could be good at is also a serious problem.