Don't Finland, China and Singapore have the highest IQs of any countries? Why can't that be the answer? To me, what you do exactly in school is likely pretty secondary.
It feels to me unlikely that PISA scores should be particularly related to ‘country average IQ’ (except by confounders like malnutrition rates) where my understanding is that much of the research has been limited or of such poor quality that the topic has become so toxic as to discourage most potential further research.
For example, PISA consistently finds substantial differences in math, reading, and science performance between countries.