Grouping by attainment is a relatively new and contested practice in Norway, where strong historical discourses of heterogeneous education are under pressure from international test comparisons, particularly in mathematics. At the same time, research indicates that Norwegian teachers have a high degree of autonomy in education policy enactment. Analysing thirteen Norwegian mathematics teachers' reflections on grouping practices, we seek to understand their decision-making processes within this context. Our findings indicate that teachers report a high degree of autonomy in grouping practices which is exercised through considerable diversity in practice and defended when necessary. Using an ecological model of teacher agency reveals how teachers explore and explain their highly situated practice and its tensions and contradictions. We find that teachers who are removed from the central locus of autonomy have less to draw on in making and justifying their pedagogic choices, while those who are more central are able to continually reassess in this highly sensitive arena. We note the implications for practice and research on attainment grouping, particularly the need for teacher autonomy and support for professionally-based local decision-making.