Grouping children by achievement levels is a thriving practice in New Zealand primary school mathematics classrooms. In this paper we look at the impact of a formative intervention project--"Developing Communities of Mathematics Inquiry"--that required a whole-school shift to mixed achievement grouping. Engeström's Cultural Historical Activity Theory framework is used to explore changes in the teachers' object/motives over the first year of the project. Teachers' learning about mixed-ability groupwork focused first on organisational and social structures, then participatory practices, and finally to students and their own mathematical sense-making. Such a shift is characteristic of expansive learning and transformative agency.