This article reports findings from the first year of a longitudinal, mixed-methods case study of a large teaching school alliance in England. This national initiative is intended to drive improvement at system level by grouping schools around formally designated teaching schools. These "alliances" work collaboratively to share learning, excellent practice and innovative ideas, principally in teacher education and development. Focused on the influence of one alliance on continuing professional development, this study identified significant leadership challenges in establishing and promoting the new alliance and its work. The policy aim of system-level improvement may be difficult to achieve practically in an environment where teaching staff lack agency in their professional development. It is suggested that school leaders need to ensure wide knowledge among their staff about professional development activity generated by a new alliance, and to ensure that such activity both meets staff needs and goes beyond existing provision.