Increasingly school leaders are being challenged to take a more instructionally focused role in their schools. This paper tracks the leadership challenges through a change process involving an assistant principal and a group of teachers, supported by a consultant, through four phases of an action research project. During the project the participants learned how to use achievement data to improve instruction for their low-achieving students. Initially, the teachers did not believe that they could influence the low literacy achievement of their students and so analyzing achievement data was irrelevant to their practice. Eighteen months later they were using the data to target their instruction more precisely and to test the effectiveness of their teaching practice and make refinements to their programs. The multifaceted challenges involved in leading such an initiative are discussed for each phase, together with conclusions about the realities of instructional leadership and the support that might be needed to undertake it effectively.