The U.S. is a big, diverse country. Some countries participating in international comparisons such as TIMSS are the size of U.S. states. I don't think one should expect that average results of the U.S. to be the best or the worst in international comparisons, and they are not.
Apologies if this news article or the study it's based on have already been discussed here, but this is interesting in that context, though based on Pisa not TIMSS:
http://www.bbc.co.uk/news/business-27442541I'm afraid that even intra-country, never mind inter-country, comparisons are extremely difficult to get solid conclusions from. There are always many more factors that vary than the educational one you want to study, so such comparisons are always subject to different interpretations.
