That's a mistake of the system in the United States. In many other countries, teachers specialize by subject in the elementary grades, the better to teach their subject effectively. See Knowing and Teaching Elementary Mathematics: Teachers' Understanding of Fundamental Mathematics in China and the United States
for a detailed discussion of elementary math teaching, or The Teaching Gap: Best Ideas from the World's Teachers for Improving Education in the Classroom
for a broader perspective on other ways to organize schools.
I agree with some points in your reply. I don't think China as a whole is well represented by the schools in its most developed urban areas. The results from Shanghai in the most recently announced test to include Shanghai surely don't reflect what students from rural areas in China would do on the same test. But even agreeing with that point, I wonder if you've had a chance to take a look at what Ma's book
says about differing classroom practices and differing lesson content between the United States and China. China is very, very, very much poorer than the United States because of the lousy policies it had in the 1950s and 1960s. But its educational policies since the 1970s have been on an increasingly sound basis, and seem to be producing admirable results in economic growth with remarkably low school budgets. But please note that I never appeal to China as a country with country-wide results that are uniformly better than those of the United States. China is especially doing well on a resources-adjusted basis, while Singapore, Taiwan, and some other countries are just plain doing well nationwide, period. (I am most familiar with Taiwan, from much time living there.)
I also agree with the idea that it's important to look at education studies "with a critical eye" and it was with that in mind that I referred fellow participants on HN on several earlier occasions to the studies showing that United States schools are underserving the most able learners,
missing opportunities to reach the top end of mathematics achievement reached by other countries. "Data doesn't lie, but analysis is often wrong and/or exaggerated," I agree, and what I find is that some forms of analysis are not even attempted by many commentators on education policy. I think writings that are good examples of good analysis
are food for thought for those of us participating on Hacker News who seek ways to improve education wherever we live.
(or its review by mathematician Richard Askey)
for example of ways American primary education could do better.