It is difficult to identify direct causal relationships between school-based professional learning initiatives and changes in the participating teachers' competencies or their students' learning outcomes. This paper contributes a novel approach to examining the impact of such school-based professional learning by integrating the constructs of complex systems and agentic practice into how the processes in curriculum re-design contribute to professional learning outcomes. The paper first presents an integrated multilayered conceptual framework that combines perspectives on complex systems and agentic practices. Next, it outlines the main levels and aspects of the complex system that need to be considered when examining school-based professional learning initiatives. Then, using a case from a multi-school professional development initiative illustrates how this framework was applied to examine the impact of a professional learning initiative focused on a collaborative science curriculum redesign to support English Language Learners (ELLs). Analysing the agentic practices that emerged within and across different complex system levels provides a comprehensive insight into how the system, constraints or amplifies particular practices.