Another anecdotal story would be in my classroom in 1980-90s Canada, where not all the students wrote those international testing exams, and of those that did,

Another anecdotal story would be in my classroom in 1980-90s Canada, where not all the students wrote those international testing exams, and of those that did, only a select few had their tests submitted by the school for grading. This was confirmed to the school's PTA (Parent Teacher Association) after concerns were raised about taking time from teaching for an exam that was not felt to help the students' learning. Having discussed the issue with a senior education ministry official in Singapore, they pre-select which schools and students write the exam. Even if all students in those schools write the exam, not all exam scores are submitted. She did a study in China recently (not yet published), and there only elite urban schools are tested, primarily in Shanghai and Beijing, with a similar selection process. I would much prefer my children to be taught in a middle-ranked Canadian school, than any I have so-far visited in Singapore or China, no matter at what level the students supposedly score on international tests.