It’s more than that.
Kids of Japanese and Chinese ancestry in USA outperform kids in Japan and China. Kids of Swedish or Finnish ancestry outperform kids from Sweden or Finland. AFAIK, this continues through pretty much all countries.
Our low performance is due entirely to our high admixture of poor-performing groups. But it is almost unthinkable to say so.
Krugman a few years back made fun of TX for their poor school performance. Iowahawk cut him/her a new one.
http://iowahawk.typepad.com/iowahawk/2011/03/longhorns-17-badgers-1.html
Blacks in TX outperform blacks in WI, latinos in TX outperform latinos in WI, and whites in TX outperform white in WI. The average difference between the states is due entirely to the differences in percentages of these groups.
A truly classic example of how entirely accurate statistics can be utterly misleading.