Improving teacher and principal effectiveness to ensure that every classroom has a great teacher and every school has a great leader is one of the top priorities of the current Obama administration’s educational reform policies as documented in its blueprint for reauthorization of the Elementary and Secondary Education Act (US Department of Education, 2010).9 Such a policy is in part grounded on some research evidence demonstrating that top-performing teachers can make a dramatic difference in the achievement of their students, and suggests that the impact of being assigned to top-performing teachers year after year is enough to significantly narrow achievement gaps (Gordon, Kane, and Staiger, 2006; Hanushek, Kain, O'Brien, & Rivkin, 2005; Sanders & Rivers, 1996). For example, Gordon, Kane, and Staiger (2006) found that students who had teachers in the top quartile of effectiveness gained 10 achievement percentile points relative to similar students who had teachers in the bottom quartile of effectiveness. Based on this estimate, they concluded that having a teacher in the top quartile for 4 consecutive years would be sufficient to close the black white achievement gap (34 percentile points).