This qualitative case study of an exceptional school in the south of England challenges the hypothesis that transformational leaders significantly impact on student outcomes. Interviews with staff and students, together with classroom observation, confirm that the head, appointed in 1995, has played an important role in transforming internal processes and in changing the context of the school. Although the observed and reported behaviour of leaders, teachers, and students matches expectations from the literature, the consequences for student achievement are unclear. Background variables seem to explain most of the apparent improvement in student outcomes. An effectiveness framework that assigns disproportionate value to examination results seems to have created a leadership paradox, where heads reported to be transformational produce only limited gains in performance. The study concludes that the government's determination to assume a strongly positive relationship between leaders and outcomes has compromised the principle of evidence-informed policy-making and that we need a different approach based on a broadly defined, qualitative conception of student success. [ABSTRACT FROM AUTHOR]