Samenvatting

This longitudinal mixed methods study, employing an explanatory sequential design, examined how in-service professional development in a secondary school context influenced teachers’ self-reported differentiated instruction (DI) practices across five domains. Particular attention was given to contextual factors within the professional development setting that may have shaped these practices. Quantitative data were collected using a one-group repeated measures design, followed by nominal group interviews for qualitative insights. Multilevel growth modeling (teacher n = 124, repeated measure n = 271) revealed significant increases in self-reported DI practices within the activities and materials domain. These improvements were linked to more engaging instructional strategies for students. No significant effects were found in the other domains. Teachers attributed the increases to peer workshops, structured lesson design with implementation support, and peer consultations. Structural constraints of secondary education, such as time limitations and departmental compartmentalization,were identified as barriers to DI development. Conversely, strong school leadership, a collaborative school culture, and a shared belief in the value of DI were seen as essential enablers. These findings underscore the importance of aligning professional development with supportive school conditions to foster sustainable instructional change in DI