While there is clear evidence that using classroom assessment for learning can improve learning significantly, research has demonstrated that it is particularly challenging for secondary teachers to change their practices in an assessment for learning direction. Many factors have been suggested as inhibiting change in these schools. Some studies do link improved student outcomes to changes in teaching brought about through professional development. They suggest that the same factors enhance professional learning in primary and secondary schools. However, the way large secondary schools are organised by subject departments exerts enormous influence on how professional learning in the sector can and should be structured. The results of three success case studies demonstrated the critical impact of school-level factors on changing assessment for learning practices in secondary schools. These factors enabled these three schools to address the departmental constraints and supported cross-curricular professional learning in assessment for learning. (Contains 2 tables.)