I taught a bunch of kindergarteners recently, and these were very well-cared for kids. Here is something scary: by age 4 there are already huge and obvious differences between kids. These are largely the product of the home environment. The rest is nature. Teachers unions are not involved. I know what our home environment is like and I can see what it does to a kid, and I can imagine what others create. The causality is palpable. And the differential results are too. From my class, I saw it. Sit still. For ten minutes. For twenty. Take turns. Read a sentence. Sound out the hard words. Write your name. Write a sentence. Draw your family. Give an opinion. Explain why. These are skills little kids can have. And there were big big differences between the kids. At age 4! If you recall the research on hockey players born in Jan vs Dec (Jan kids are bigger, better young, and go pro more often), then you will appreciate the massive compounding effect of being called very smart at age 4 vs being "nice". Good luck to the educators who seek to make Nobelists of all these bus drivers' kids in public schools (though bus riders' kids may be hosed too).