This article focuses on how assessment practices are used by teachers to develop and maintain their own professional standing -- how assessment works for them as professionals and the work they must do with it to be successful. Reporting on an empirical study involving interviews with 12 primary Key Stage 2 (7-11 years) teachers and using an analysis focused on teachers' accumulation of capital through their work, the paper examines how teachers construct assessment data in particular, often carefully managed, ways. This, in turn, subjectifies both pupils and the teachers themselves in ways which create tensions in practice. The paper argues that high-stakes accountability through assessment is unlikely to be helpful in two ways: firstly, it does not actually do what teachers claim in accurately measuring pupils' progress; secondly, that it is likely to lead to pedagogy that has negative effects on them as learners.