From the halls of Congress to the local elementary school, conversations on education reform have tossed around the term "achievement gap" as though people all know precisely what that means. As it's commonly used, "achievement gap" refers to the differences in scores on state or national achievement tests between various…

States are engaged in serious efforts to raise student achievement, paying particular attention to narrowing the achievement gap between high- and low-performing students that has persisted in public schools for decades. The achievement gap is particularly troubling because, all too often, those at low achievement levels are minority, special…