Regarding reasons for the gap, the article dwells on one hypothesis, commonly called the investment theory: richer families have more money to invest in their kids. (The article might have mentioned that richer families not only have more financial capital, but more human capital and social capital.) The article does not mention at all another major theory of the economics of educational achievement; stress theory. Kids (and parents) who live in poverty live under systemic stress. A great deal of research in the last ten years has shown that this stress has direct cognitive consequences for kids, and also affects how parents treat their kids. (Any parent knows that you’re not at your best when you’re stressed.) An open-access review article on this research can be found here.
Another important point the article misses concerns what might be done. It ends with a gloomy quote from an expert: “No one has the slightest idea what will work. The cupboard is bare.”
I think there is more reason for optimism, because other countries are doing a better job with this problem than we are. The OECD analyzes the PISA results by reported family SES. In virtually every country, high SES kids outperform low SES kids. But in some countries, the gap is smaller, and that’s it’s not just countries that have smaller income gaps.
Economic inequality within a country is often measured with a statistic called the Gini coefficient which varies from 0 (everyone has the same net worth) to 1 (one person has all the money, and the other has nothing). Rich children score better than poor children in countries with large Gini coefficients (like the US) and the rich outscore the poor in countries with lower Gini coefficients (like Norway). Being poor predicts lower scores everywhere, but the disparity of wealth means more in the US than it does in other countries. What’s significant is that the relationship between income and test performance is stronger in the US than it is in most countries. (The US has the 3rd strongest relationship between income and student performance in Science and 10th highest for math, in the 2006 PISA results).
Some countries, (e.g., Hong Kong), despite an enormous disparity between rich and poor, manage to even the playing field when the kids are at school. The US does a particularly poor job at this task; wealthy kids enjoy a huge advantage over poor kids. People generally argue that the US is different than Hong Kong, we’re a large, heteroogenous country, and so forth. All true, but the defeatist attitude won’t get us anywhere. We need more systematic study of how those countries solve the problem.