> > PISA score is a form of grading, is it not? > No, it's an independent test.

I'm sorry. I'm not a native speaker. Does the word "grading" pertain to only tests that are a part of school curriculum? I thought it could mean assigning a score to any test, even independent, even not related to education at all.

> Correct. Students take the PISA in their native language.

Right. So assuming that scores of the students are averaged out together across all test takers in the country and some of them learned to read French rather than English they might skew the average. So comparing scores of Canada and UK for the purposes of comparing how well English learning goes in those two countries might not be valid approach.

We should be comparing UK with sub-population of Canadian students that took the test in English. Not sure if PISA provides such data.

I probably can't object to comparing UK and Ireland on that grounds. Or do some students take PISA tests in Irish there?