For several decades, there has been a global trend of the devolution of autonomy to local leadership levels. The topic has consequently gained attention in educational research, showing the complexity and multi-layered nature of the phenomenon. Nevertheless, empirical research has focused largely on individual leadership or institutional school autonomy, overlooking school leader unions’ collective autonomy at the intersection between leadership levels and as a link between policy and practice. This paper aims to explore the communication and formation of collective autonomy through school leader unions in their mediating role between policy and practice, under shifting national policy contexts with increased global policy influence. Attention is drawn to union magazines and how they have depicted the issue over time, analysing texts on assessment from nearly 300 issues through a two-fold conceptualisation of collective autonomy as the capacity to influence policy and control individual professionalism. The results highlight the possibility of unions functioning as mediators between policy and practice, national and local, when collective autonomy is not too restricted. The study calls for further empirical research on collective autonomy, paying attention to the intricate relationship between policy, collectivism and individual leadership practice.