In the largest in-depth study of school leadership to date, the Wallace Foundation's report, "Learning from Leadership: Investigating the Links to Improved Student Learning," gathers and analyzes quantitative data confirming that education leadership has a strong impact on student achievement, as measured by student test scores. The study shows that leadership makes its mark largely by strengthening a school’s “professional community” – an environment where teachers work together to improve classroom instruction. It also finds that rapid turnover of principals reduces student achievement. In addition, the study shows that although the principal remains the central source of leadership in schools, he or she is far from the only source. Indeed, the highest performing schools operate by a “collective leadership” that involves many interested players – including parents and teachers – in decision-making.