This paper considers the effect of recent education policy on assessment practices in English primary schools and how these affect relationships between teachers and pupils in an increasingly market-driven school system. Previous research has focused largely on the effect of markets at a systemic level, but less attention has been paid to how marketisation plays out in teachers' work at classroom level. Similarly, research on assessment has tended to examine teachers' practices in relation to pupils' learning rather than examining it in terms of the role it plays in teachers' professional working lives. This paper brings these ideas together, using the latter as the context for understanding the former. In particular, it focuses on recent policy changes to the way schools in England are evaluated, moving from final attainment to pupils' progress "in-year", and considers their potential for altering the dynamics "within" schools rather than just between them. It draws on a small-scale, interpretive, empirical study involving interviews with primary teachers in England. Data from this study are used to illustrate how tensions can arise for teachers and how internal market competition can be set up between them in which pupils' achievements become a commodity to be made use of, encouraging teachers to consider pupils' achievements as private, rather than public, goods. The paper thus provides a starting point for exploring the effect of market-orientated education policy on teachers' assessment practices at the in-school level and briefly examines some of the implications of this effect.