Numerous studies reveal the tremendous impact teachers have on student achievement. For example, there is a 39 percentage-point difference in achievement between students with “most effective” and “least effective” teachers. In classrooms headed by teachers characterized as “most effective,” students posted achievement gains of 53 percentage points over the course of one academic year, whereas in classrooms led by “least effective” teachers, student achievement gains averaged 14 percentage points, according to a report by McRel, a national education development and research non-profit organization.

This research on teacher quality has spurred a national effort to rethink how we prepare, support and evaluate teachers, according to Kerri Briggs, Jacquelyn Davis and Gretchen Rhines Cheney in an article for Education Week. "But what often gets lost in the policy conversation," they argue, is the role of the school leader in ensuring that there is a strong teacher in every classroom. Without a high-quality principal at the helm of a school, students are unlikely to have successive years of effective teaching. Principals are best positioned to ensure that every student has a great teacher year after year. We also know that strong teachers will leave a school if they do not feel the principal provides a supportive environment."

The data shows that leadership makes a difference. A top principal leads the average student to learn 0.05 standard deviations more annually than he or she would in a school with an average principal, according to a recent paper from the National Bureau of Economic Research.

But good principals are hard to find. Nearly two-thirds of principals reported that their preparation programs had not prepared them for the realities of leading a school, according to data from a 2006 Public Agenda Survey. Lack of preparation could contribute to the fact that almost 50 percent of principals leave the field within the first five years of starting, with a majority leaving within the first three years, according to research by Stanford professor Linda Darling-Hammond.