In recent years, the concept of Assessment for Learning (AfL) has travelled across countries, giving rise to a range of educational policy initiatives and school development projects. While researchers have focused on issues such as how formative assessment can support student learning and lead to more efficient classroom practices, less attention has been paid to the collaborative work required to develop shared assessment practices at the school level, and to the integration of AfL-related principles and tools in the collective practice. This paper focuses on the challenges emerging for a teacher team engaging with an AfL project in a lower secondary school in Norway. By employing concepts and perspectives from social practice theory, AfL is understood as a problem complex that needs to be explored and developed locally. We have analysed data from seven video-recorded meetings of an AfL team, supplemented by interviews and field notes, in order to make visible the micro processes of teachers' collaborative work. The analysis shows how the teachers needed to rework historically established practices and principles, which in turn called for a negotiation and re-contextualisation of new concepts and artefacts in their efforts to develop shared assessment practices. This again required ways of representing existing practices and imagining future scenarios. Based on these observations we recommend that AfL projects be understood in their wider curricular, institutional, and social contexts, and that the constructive dimensions of teachers' collaborative work in such projects should be acknowledged.