The achievement gap refers to the gap in educational outcomes between rich and poor kids (and sometimes among white and non-white students). The education reform movement is all about closing that gap. I am skeptical that marginally altering the schooling environment will do much on that front, but we can leave that skepticism aside for this post. Often, education reformers propose schooling changes that are supposed to improve educational outcomes. For instance, lengthening the school day and the school year has been shown to improve outcomes. Because these proposed changes improve educational outcomes, it is generally taken for granted that they can be used to close the achievement gap. But this does not actually follow. The achievement gap describes the relative educational outcomes of rich and poor kids. As such, schooling changes that increase educational outcomes in general do not necessarily alter the achievement gap. For instance, suppose lengthening the school day improved educational achievement by 5% across the board. If implemented universally, that would mean rich and poor kids get the 5% bump. They would both be better off than they were before, but their relative positions will not have changed, and the achievement gap will not have narrowed. To close the achievement gap then, a proposed educational change must have two features:

1. The change improves educational outcomes.
2. The change improves educational outcomes of poor kids more than it improves educational outcomes of rich kids.

It most cases, it seems like requirement two will not be met. If some schooling innovation is found that improves outcomes, you can be sure the schools that serve rich kids will generally seize upon it as well. The only way that the innovation could close the gap is if, by its very nature, the innovation boosted poor kids more than rich kids. I can imagine some cases where an innovation might do that, but I can imagine many more where it will just benefit kids' education in general. If we assume, as is reasonable, that rich kids will always find a way to gain access to any of the innovations that come out of the education reform movement, then it becomes clear that educational innovations alone will not close the gap. If you take two kids who are otherwise identical except one is very poor and the other is very rich, the poor kid will just do worse in school period. At minimum, any argument about something closing the achievement gap should proceed on the two-step basis spelled out above. If someone merely shows that proposed schooling change improves outcomes, they have not done enough to explain how that will close the gap. They must do a second bit of analysis where they explain how the improvement will be greater in poor kids than the rich kids who will also presumably benefit from it.