Hi. I listened to a programme on Radio 4 that discussed the Pisa testing system and what I took from it was that although the data is interesting, it isn't really a fair comparison as children in different places learn to deal with the tests in very different ways.
Apparently there is lots more data amassed along with the tests and one of the things learnt from the data was that British children were generally very happy at school, which should not be overlooked. I'm not a statistician, but it's worth reading a bit more about the Pisa tests before getting too depressed about our schools!