Purpose: Applying an analytic model to better understand collective leadership development, this study examines three high schools: one urban, one suburban, and one rural. Each school's unique structure and context tests the model's explanatory power. Research Methods: Using a multiple-case study design, data consisting of interviews with teachers and administrators (n = 64), document analysis, and observations were collected from each of the three high schools to describe and explain variation in collective leadership development, practice, and student outcomes. Findings: Schools' efforts to develop leadership are organized for cross-case analysis by model constructs. Variation between schools exists in collective leadership capacity, practice, and student outcomes. This variation is explained by antecedent factors that include principal support of teacher leadership, initial teacher capacity, school conditions, work design, and leadership development experiences. Specifically, catalytic principal support, relational trust, professional capital, and views of leadership as work enhance development. Moreover, because of the iterative nature of improvement captured by the model, improved collective leadership practice influences the antecedent constructs. Implications: This study has implications for how schools and districts understand collective leadership development efforts of administrators and teachers. The analytic model facilitates the examination of school contexts to understand the potential and relative success of development efforts.