You've pointed to a study that found a link between individual's grades and genetic differences in one country. But it's a big leap to go from there, and explain why a particular school would have bad grade averages, especially schools in different countries - unless you're making the argument that schools are composed of people from large groups of genetically similar people, and these people are necessarily very distinct from people at schools that have better grade averages. Intuitively, that seems unlikely to me; the society would have to be extremely stratified, geographically and genetically, with very little intermixing.