I can't speak about American schools, but what I can say about primary schools in Australia is the diversity of competence of students in the class room is profound. I've experienced 5th graders with a reading level similar to advanced preppies (year before year 1). How teachers handle this is beyond me.
I will note that primary school teachers from China have said our class sizes are too small, and the class contains too many distracting visual cues. I can't see any other alternative to STEM becoming an Asian endeavor, our school systems almost makes that inevitable.