Purpose: This study aimed at exploring mathematics teachers' reactions, learning, school leaders' support, and the use of the lesson study for transformative assessment (LSforTA) program ideas in practice. Design/methodology/approach: The LSforTA program was new and therefore, a local and grounded approach was needed to examine teachers' knowledge and their skills acquired using LSforTA. A design-based research approach was therefore selected to evaluate and refine the LSforTA approach. Findings: The program affected teachers' practices of transformative assessment and enhanced their knowledge and skills in assessing students transformatively in a positive way. The paper concludes how LSforTA procedures were adapted in response to this evaluation and provides suggestions for future development and research. Originality/value: With its original combination of conceptual and theoretical lenses, the research contributes to the academic literature by linking transformative assessment, lesson study and school context. This connection provides new opportunities for teachers to develop strategies to create meaningful assessment practices embedded with their instructional process in the context of their schools.