It's an unfair comparison. In the US, we put ALL children into schools, regardless of aptitude or ability. This isn't true across the world. At fifteen, if you're failing in school, we generally won't LET kids drop out, even if they're clearly not able to deal with that kind of academic environment and we're probably just battering their egos for no particularly good reason.
These comparisons always annoy me. They should be shown side-by-side with total population enrollment rates by age.
Not that I'd really trust anything that was reported by China. Not so much for the allowance of dissenting opinions over there. The US does sketchy-ass stuff with personal freedoms, but, for now, China's still got us pretty well beat on the oppression front.
Also re: SES having an effect: Oh hells yes it does. Poverty is a huge stressor, and stress tends to fuck with the way your mind works. (And I say this very broadly. Poverty also has a whole bunch of stuff that goes along with it, as precipitating factors and as consequences, which would be likely to disadvantage students.) This isn't to argue that "poor people are stupid." But, shit is just plain generally fucking HARDER when you're poor, and that includes school.