If your children are about 15 years old or more, you could ask them to try out the sample questions and see how they fare. Teachers may also like to try them out. It would be interesting to find out what teachers think of the level of these questions and the capability of their students to answer them.
The list has sample questions from the field trials and the actual PISA tests administered in 2000, 2003 and 2006 including questions testing reading literacy, mathematical literacy, and scientific literacy. The detailed answer keys to all the questions are also provided.
For some of the questions used in the actual PISA tests in past years, country-wise data on the percentage of children that got the answer right is also provided.
I think students studying in English medium schools in India would find the questions that test reading literacy quite challenging. In many English medium schools in India I doubt if the students are as fluent in English as they ought to be, despite English being the medium of instruction. I haven't come across the samples of questions adminstered in Tamil or Hindi during the 2009+ cycle of tests in Tamil Nadu and Himachal Pradesh. It would be interesting to look at samples of those. But in any case, the average test scores of those children in India who took the PISA 2009+ tests in their own first langauge were a bit lower than those who took the test in a language different from their first language (presumably, English was the language the latter took the test in). Reading literacy may be an issue even for students in schools where the local langauge is the medium of instruction.
If reading and comprehension is a problem, then students are likely to find the questions that test mathematical and scientific literacy challenging too, even if they may be good at math and science.
We may be failing to build reading and comprehension skills as well as we ought to, whether in the local langauge or English. We need to figure out why we are not doing well on that front and what we can do differently to build those skills. Lack of reading and comprehension skills may be one of the reasons for the poor show in the PISA 2009+ cycle of tests by students in Tamil Nadu and Himachal Pradesh.
Aside 1: You can also try out examples of computer-based questions to test reading of digital texts, mathematical and problem solving skills. These have been tried out in a few countries during the 2009 cycle and will be tried out in the upcoming 2012 PISA cycle too, in some countries.
Aside 2: While only Tamil Nadu and Himachal Pradesh participated in the 2009+ cycle of PISA tests, more states in India will be participating in the 2012 cycle of PISA tests.