Maybe US students' test scores are lower than other countries' simply because students in the United States just don't care about school performance as students elsewhere. Maybe a fundamental societal difference is primarily responsible, not necessarily bad or underfunded schools. I know that when I was in school, I got the impression that not too many people cared about how well they did, or school in general. Even kids in the honors classes were often only there because their parents insisted on it.
It doesn't even have to mean that kids in the United States don't believe in or strive for personal excellence. It could just be that in the US, people are less likely to corrolate good grades with that. I know that I tried to ensure that the grades I got in school had little to nothing to do with my own self-image.
Note that I've spent about three minutes thinking of this theory.