Obviously a better school assigns more homework and there's a feedback effect but still...
EDIT: definitely wasn't trying to make a statement reading into race or anything, just trying to point out that it's hard to entirely blame "crappy schools" at a reductionist level... that blog actually has a lot of really interesting stats. He claims in one article (with specific data to back him up) that lower US scores are basically a product of simpson's paradox - our non-minority students score better than other countries' non-minority students and our minority students score better than other countries' minority students (whatever the minority for those countries happen to be), but we have a lot more minorities than other countries so our overall average is worse. He backs that up with data comparing the scores of immigrants vs country of origin - Swedes in the US score better than ethnic Swedes in Sweden, Mexicans in the US score better than ethnic Mexicans in Mexico, etc. It's all kind of interesting, actually.
Becker and Posner had a bunch of stuff on PISA also, which was interesting, as well.