Secondary schools in New Zealand use assessment data for school self-evaluation, but little research has explored exactly how schools are using these data. This case study of selected high schools explored the perspectives of teachers and school leaders whose schools had recently implemented a student assessment tracking and monitoring "traffic light" tool. Informed by a realist approach, the study involved a series of three focus groups followed by individual interviews with 13 educators at four secondary schools that had been identified as effective at school self-evaluation. The results highlight that data use processes led to changes in practice in teachers' work and contributed to structural changes in these schools. This study shows that data use can be a powerful force, with the potential for good, but it also raises some concerns about the unintended consequences of the use of assessment data.