There is a dearth of empirical evidence that examines the impact of teacher professional development for culturally responsive pedagogies, particularly on Indigenous student achievement and teacher practices. Te Kotahitanga was a large-scale professional development initiative for culturally responsive practices for secondary teachers in New Zealand. To study its impact, we used a mixed-methods research approach to gather and analyze data on student achievement outcomes, classroom practices, and perceptions of teachers and students. While results suggested positive changes associated with the program, findings also highlighted ongoing challenges associated with transforming practice for Indigenous Maori students. Specific challenges of analysis are highlighted, along with recommendations for further research and development work in secondary schools.