What's to be done about those gaps in parenting skills? The parents aren't crack addicts, so social services will never get involved. This is where the schools have to step in. They have to level out these differences. All day nursery schools. Free books for toddlers.Towns need to offer parenting classes and organize babysitting cooperatives. Churches have to organize parent groups.
The problem is, parents who let their kids cut summer school probably aren't going to force them to go to all day school programs. Or read to them. Or show up for parenting classes. The parents are choosing to let the kids do what they want either because they don't value school, or because they are too stressed or exhausted or possibly too lazy to engage in the confrontation and micromanagement required to force their children onto a different path.
High income parents do these things because a) they view them as their own path to success b) their social circle values these activities, and punishes parents who do not do them and c) people with more satisfying jobs have more emotional energy for the unpleasant work of parenting--they have room left in the mental "chore" basket. I don't know what sort of social program can change any of these factors.