Despite calls for evaluation practice to take a complex systems approach, there are few examples of how to incorporate complexity into real-life evaluations. This article presents the case for using a complex systems approach to evaluate a school-based intimate partner violence-prevention intervention. We conducted a post hoc analysis of qualitative evaluation data to examine the intervention as a potential system disruptor. We analysed data in relation to complexity concepts particularly relevant to schools: 'diverse and dynamic agents', 'interaction', 'unpredictability', 'emergence' and 'context dependency'. The data-two focus groups with facilitators and 33 repeat interviews with 14-17-year-old students-came from an evaluation of a comprehensive sexuality education intervention in Mexico City, which serves as a case study for this analysis. The findings demonstrate an application of complex adaptive systems concepts to qualitative evaluation data. We provide examples of how this approach can shed light on the ways in which interpersonal interactions, group dynamics, the core messages of the course and context influenced the implementation and outcomes of this intervention. This gender-transformative intervention appeared to disrupt pervasive gender norms and reshape beliefs about how to engage in relationships. An intervention comprises multiple dynamic and interacting elements, all of which are unlikely to be consistent across implementation settings. Applying complexity concepts to our analysis added value by helping reframe implementation-related data to focus on how the 'social' aspects of complexity influenced the intervention. Without examining both individual and group processes, evaluations may miss key insights about how the intervention generates change, for whom, and how it interacts with its context. A social complex adaptive systems approach is well-suited to the evaluation of gender-transformative interventions and can help identify how such interventions disrupt the complex social systems in which they are implemented to address intractable societal problems.