The article stops short of an explanation, because the explanation for the gap may actually be very simple. However, it is more fashionable to talk about inequality gaps without actually looking at the causes.
Poorer students are more likely to be lazy and not want to study, due to their upbringing and because their parents are also less educated and have more distrust in the skills that are taught in schools. Richer students' parents are more likely to be well-educated and thus to impose on their children that they need to study harder and take education more seriously.
This is not news or any revelation. Of course, some of the richest kids may also be lazier, but the big difference would be between the poor and the middle-class kids.
Poorer students are more likely to be lazy and not want to study, due to their upbringing and because their parents are also less educated and have more distrust in the skills that are taught in schools. Richer students' parents are more likely to be well-educated and thus to impose on their children that they need to study harder and take education more seriously.
This is not news or any revelation. Of course, some of the richest kids may also be lazier, but the big difference would be between the poor and the middle-class kids.