Sheer size has to play a major factor here. Comparing the US to city-states like Hong Kong and Singapore or relatively tiny countries like Finland is really suspect because there is so much difficulty in the managing and improving an education system as large and diverse as the US has.

Unfortunately, there isn't a great country to compare to: the countries with the closest populations are Indonesia, Brazil, Pakistan, Nigeria, etc. and comparisons there have other problems. I'm guessing the best possibly comparisons are probably Brazil or China.