The US has a much bigger spread than any other country (the largest standard deviation of wealth in the developed world).
Our overall scores are unspectacular because we have a high percentage of children living in poverty, over 20%. This is the highest among all industrialized countries. In contrast, child poverty in high-scoring Finland is less than 4%. For Cleveland and for the US as a whole, the major problem is poverty. Before we worry about teacher quality, institute longer school days, and increase testing, we need to make sure that all children are protected from the effects of poverty: This means adequate health care and nutrition, and access to books. When we do this, American test scores will be at the top of the world.-- Stephen Krashen