The introduction of new assessment policy can reverberate throughout all levels of schooling. This paper presents an in-depth investigation into one school's response in the Junior years of secondary education to the introduction of a new Senior years' assessment system. The investigation focuses on the educational context of a high-performing school where school leaders decided to prioritise learning rather than summative assessment in the Junior school. The findings reveal the practices required to make critical changes to the assessment culture and related assessment systems for the school, within a context of national assessment accountabilities and high community expectations. These practices include the collaboration of school leaders with teachers, students and parents to drive change while maintaining high performance.