I would chalk this up to the costs of increasing inequality, but the author does not seem to do so. Instead he says that:
although rising income inequality may play a role in the growing income achievement gap, it does not appear to be the dominant factor. The gap appears to have grown at least partly because of an increase in the association between family income and children’s academic achievement for families above the median income level: a given difference in family incomes now corresponds to a 30 to 60 percent larger difference in achievement than it did for children born in the 1970s.
So for an identical difference in family income now and in the 1970s, the richer kid today is doing relatively better than the richer kid in the 1970s.
Perhaps the weirdest thing is in another article which notes:
Greg Duncan and Katherine Magnuson note in chapter 3 of this volume, the income achievement gap is large when children enter kindergarten and does not appear to grow (or narrow) appreciably as children progress through school.
So all the difference is in the first 5 years of life? Those are obviously some important five years, but it does beg the question of whether this is measuring ability differences rather than educational differences. Is this a result of the meritocracy and genetic sorting? I hope not.