What always fascinates me about this debate is how much cherry-picking of statistics is involved. In all cases, someone in the media or on a blog, cherry-picks some statistic out of the PISA test, then writes a headline like "Oh no! The US is falling behind and we're DOOMED!"
I've actually read the results of the PISA test. The results are surprising. The US is approximately average among the OECD countries, virtually indistinguishable from France, Germany, or the UK. Even the vaunted German education in science, is only modestly better than US education in science: 539 vs 502. Even Japan, which has a reputation for non-stop studying and cram schools and so on, scores 539 on science, vs 502 in the US. I'm using science as an example because it's the middle case: the US performs slightly better relatively on reading, and slightly worse on math, but not to any significant degree.
Most industrialized first-world countries are not very different from each other on the PISA test. China is much better, however China is widely known to cheat on this test, and they cherry-pick students from an elite high school in Shanghai rather than randomly from the population, so the Chinese results were prefaced by an asterisk on the PISA results until recently. Aside from the Shanghai Chinese results, most industrialized countries are not very different from each other. Take the science test as an example. Spain performs very poorly, at 489; and Japan performs very well, at 539. Almost all large, industrialized countries are within this range. There are one or two outliers (Finland is an example) but not many.
The only way in which the US educational system is demonstrably inferior to any other large, industrialized country is the proportion of students who score a 6 (the top score) on the math test. In this regard, a few countries (like Japan, Switzerland, and Korea) have ~7% of their students which score in the very top category of the math test while the US (and most other countries) has about ~2%. This is the only worrying statistic. China (Shanghai) has a fantastic score in this regard, but again it is cherry-picked.
The lesson of the PISA test is this: most rich countries are quite close together in almost all regards. However a few of them (Japan, Korea, Switzerland, and Taiwan) have a small portion of their populations (less than 10%) who score very well in math.