Concern about the behaviour of boys and young men is generating a great deal of debate in Britain. Within education, a 'common-sense' view of boys' behaviours tends to rest upon biologically or socially essentialist understandings. Using theoretical approaches based on social constructionist and post-structuralist understandings of identity formation, this paper analyses empirical data collected during qualitative research in a British primary school. The paper argues that male identities are neither normative nor biologically or socially reproduced. Instead, they are best understood as fractured and shifting, as positionings within discourse. The various masculinities found in the classroom are hierarchically organized, with class playing a structuring role. The boys themselves, as well as the curriculum and the community, are all actively engaged in constructing the particular masculinities in the classroom. This occurs within a framework of educational reform that acts to privilege some masculine modes above others. The paper concludes that an understanding of a multiplicity of masculinities, constructed in the interplay of formal school curriculum, informal pupil cultures and discourses of masculinity, provides a more fruitful basis for analysis and action.