No topic in education has received such public and professional attention as class size. Calls for reductions in class sizes are rallying points for parents, teachers, and administrators across the nation, and politicians have rushed to claim credit for introducing policies aimed at reducing class size. The pupil-teacher ratio in a school district, for example, is frequently used as the fundamental metric for quality, and comparisons across districts become indexes of equity. A prime reason for the attention to class size is that it represents such an extremely convenient policy instrument, one amenable to general political action. A legislature or a court, wishing to alter student outcomes, can easily specify a change in a class size, whereas other potential changes are much more difficult to effect.