The digitalisation and datafication of education has raised profound questions about the changing role of teachers' educational expertise and agency, as automated processes, data-driven analytics and accountability regimes produce new forms of knowledge and governance. Increasingly, research is paying greater attention to the significant role of digital intermediaries, 'in-between' edtech or State authorities and the classroom itself, in educational transformations. School data infrastructures, understood as comprising diverse sociomaterial elements including teachers, data, software, standards and pedagogical practices, is one such intermediary through which teacher expertise and agency is reconfigured. In this paper, I focus on teachers' involvement in processes of data infrastructuring in which people, platforms, systems and tools come together to create, enable and maintain data flows. Drawing on a sociomaterial ethnography of a secondary school in England, I analyse the work of a school data office in the behind-the-scenes work of data infrastructuring. The findings detail the significant labour and expertise involved in data infrastructuring, the dynamic, expanding and bespoke nature of the school data infrastructures that emerged, and processes of decontextualising and recontextualising numbers. The paper argues that the work of data infrastructuring undertaken by and through the school data office was an intermediary process which worked to both de-professionalise and re-professionalise teachers in new ways. In the process, this created new kinds of educational data experts and expertise, who gained significant influence and power within and beyond the school, both challenging and reinforcing existing organisational and governing power flows.