The current study reported here is one within a research project aimed at the identification of enabling and constraining factors in a two-year school-development project at a large secondary school in Sweden, where all teaching staff were involved in improving the quality of instruction through collaborative analyses. In this project a development group, consisting of one principal and eight lead teachers/middle leaders, played a vital role. Based on activity theory and an understanding of leadership as practice involving individuals, organization and artefacts, this study sets out to deepen the knowledge of leadership practices in locally situated, teacher-driven, school-development work. Organizational changes occurring when the development group sought to achieve a model for systematic collaborative learning are analysed, with a specific focus on the role of middle leaders. Data were collected through observations and interviews during the project’s planning phase and through subsequent interviews and continuously written self-reflections during its operational phase. Several contradictions on various levels in the activity system are identified, and it is suggested that the school’s way of organizing teacher-driven school-development work – by transforming the rules, division of labor and mediating artifacts of the activity system – enabled collaborative learning and analyses of instruction that involved all teachers at the school.