Internationally, assessment and the use of diagnostic data are recognized as critical capabilities for teachers. This is not a recent development, with assessment recognized for some decades as playing a significant role in informing learning and learners.This paper will examine whether teachers and members of the school leadership team utilize the "National Assessment Program -- Literacy and Numeracy" (NAPLAN) data for informing teaching and improving learning. Using a theoretical framework that draws on the conceptualisation of assessment as a social practice and Wenger's social theory of learning as a shared enterprise in a community of practice, this paper will provide evidence of the common power relationships (school leaders and teachers) that exist as part of social structures within a community of practice. The paper will present how these power relationships impede access to, use of or enable expertise in the analysis of NAPLAN data.The findings are based on an empirical study of nine case study schools across two Australian states and suggest there are inequalities of access to NAPLAN data between school leaders and teachers. The study discovered variegated access and distinct pathways of data dissemination and analysis dependent on a teacher's specified role within the school. The paper concludes with suggestions for greater stewardship from school leaders in building school cultures of data literacy and highlights the importance of collaboration between school leaders and teachers to build professional capability in this area.