In 1989, New Zealand started to follow an international trend of reforming education policy according to the neoliberal principles of competition, choice and self-managing schools. Since then, the increasing availability of digital data in schools has corresponded with the development of student achievement measurement tools and benchmarking of standards that enable comparison of schools and cohorts of students. More recently, national policy targets for student achievement have been introduced and form the basis of accountability measures. The article uses Hargreaves and Shirley's 'Four Ways' characterisation of education policy change as a framework to examine the influence that national policy has had on the use of data, on power relations between schools and the national policymakers, and on the challenges faced by school leaders. Interviews in 16 schools explored the types of data available in each school, how they are used and how principals, as leaders in these self-managing schools, would like to be able to use the data. Two systemic influences explain the patterns found in the research. The first is the tension principals face between data required for accountability reporting and data needed for school-based decision-making. The second is the issue with regard to economies of scale and marketisation of education that affects equitable access to the knowledge, tools and expertise that enable effective data usage.