In all countries, schools are sampled, attempting to control for some variables. Around 5000 students, 160 schools in the US. Note that schools may/may not choose to participate, and the same with students. PISA has minimum standards for acceptance as sampled selection, in an attempt to avoid the obvious bias that countries would have an interest to produce in their samples.
http://nces.ed.gov/surveys/pisa/faq.asp
Given the optional sampling, and obvious incentives, I'm skeptical that this is particularly accurate, and an apples to apples comparison.
Also, homeschoolers, one of the strongest demographics for ethnic and intact family reasons, are likely missed by this sample, skewing results in the US down. They appear to be about 4% of the total population, disproportionately white and in two parent households.
Note that schools may/may not choose to participate, and the same with students.
They control for schools opting not to participate; see the section under substitute schools.
It's standard research ethics that minors (and their guardians) be given the option to refuse to participate in a study.
...Also, homeschoolers, one of the strongest demographics for ethnic and intact family reasons, are likely missed by this sample, skewing results in the US down. They appear to be about 4% of the total population, disproportionately white and in two parent households
Post will be returning in Main, after a rewrite by the company's writing staff. Citations Galore.