That's the verdict from the National Center for Education Statistics on the US results on the Program for International Student Assessment (PISA). Every three years, countries around the world have thousands of 15-year-olds participate in this internationally benchmarked exam. The idea is to see how students stack up on an international basis.
As usual, the US is not exactly on top of the heap. 15-year-olds boosted their scores in science, coming in right at the middle of OECD countries, while in math, US students score a bit lower. (In reading, the US is above average for OECD countries, so that is good).
What's interesting to me is that our mediocre scores are not the result of having a heterogeneous population, including many English language learners. A McKinsey study that came out a little over a year ago found that the top 10% of US scorers on the PISA don't stack up well against the top 10% in other countries either. One thing we do stack up well on? We spend a lot per pupil. For years, America has been focused on inputs (funding, small class sizes, etc.) rather than outputs, and these international comparisons always show it.