Timss is the Trends in International Mathematics and Science Study. This article provides a good overview of the results.
In fourth grade math, the United States ranks 11th. Eight countries had a score that was significantly higher than the US, including Hong Kong SAR, Singapore, Chinese Taipei, Japan, Kazakhstan, Russia, England, and Latvia.
In eighth grade math, the United States ranks 9th. Chinese Taipei, South Korea, Singapore, Hong Kong SAR, and Japan had scores that were significantly higher than the US.
In fourth grade science, the United States ranks 8th. Singapore, Chinese Taipei, Hong Kong SAR, and Japan had significantly higher scores than the US.
In eight grade science, the United States ranks 11th. Nine countries had significantly higher scores, including Singapore, Chinese Taipei, Japan, South Korea, England, Hungary, Czech Republic, Slovenia, and Russia.
In the last decade, the US has made impressive increases in math scores, but science scores have remained flat.
Update: For the British perspective, this article, entitled “British pupils lead the western world at maths – but Borat’s glorious nation of Kazakhstan puts us to shame,” is well worth a read.