I agree it's worth looking into, potentially very emphatically. But that means doing some hard science.

That means don't just look at test scores by county, it means looking at DELTAs in test scores across time by numerous dimensions (income, poverty, did the kids get breakfast, school, etc) and an open mind to every possibility. Perhaps comparing against the parents' test scores might help elucidate.

Given the rise of so many unexplained mental health phenomena, I personally suspect there are environmental factors, but again, we need hard science and a lot of it.