You cannot understand modern education policy without a grasp of the achievement gap, says Jay Mathews, columnist for the Washington Post. On average, low-income students have lower academic achievement than affluent students. Black or Hispanic students similarly score lower on standardized tests, on average, than white or Asian students. School leaders say they want to reduce those gaps but are uncertain about how to do it. They should read a new book by Arlington County educators who mounted one of the most sustained assaults on the achievement gap ever seen in this area. The book is “Gaining on the Gap: Changing Hearts, Minds, and Practice,” [1]by Robert G. Smith, Alvin L. Crawley, Cheryl Robinson, Timothy Cotman Jr., Marty Swaim and Palma Strand. The main man is Smith, the Arlington school superintendent from 1997 to 2009…