Not entirely unrelated: In some of these countries, school is mandatory for 15-year-olds (e.g. the Netherlands). In other countries, like the US, you can drop

Not entirely unrelated: In some of these countries, school is mandatory for 15-year-olds (e.g. the Netherlands). In other countries, like the US, you can drop out by (or before) that time. I wonder if this skews the results in any way. In the NL, a random sample of 15-year-old students is essentially a sample of all 15-year-olds. In the US, it's a sample of all 15-year-olds who are still in school; in other words, you are not testing the results of kids who have dropped out. If you did, it seems to me that this would likely bring the overall score down.