I don't think the authors are calling for more spending. In fact they point out that funding is not a cure-all:

> In 1985, based on the theory that funding would close education gaps, a judge ordered enormous spending increases in Kansas City Schools, tripling the district’s budget and enabling them to run through a wish list of everything they could dream of to close the gaps. They built new schools, created a busing plan, and reduced the student-teacher ratio to a record low nationwide, throwing money at the problem for more than a decade. But when the Supreme Court ordered a reversal in 1995, the district’s test scores had not improved, its black-white gap had not closed, and it was no more integrated than when it began.

The piece focuses more on encouraging separation of pupils by ability, instead of uniformly teaching to the median.