Formative assessment (FA) is an important tool for supporting student learning. It provides learners with information about their learning process and supports them through feedback on how to improve their learning. Although it is recognized by secondary school teachers as crucial to language assessment, the integration of FA in their second language teaching practice has been limited. A participatory action research (PAR) design in the present study offers insights into how the learning process of teachers implementing FA can be encouraged. Teachers involved in this PAR were offered an FA toolbox and the opportunity to learn from their peers as sources of support. The data analysis of semi-structured interviews, action plans, and video coaching sessions shows that the participants react to this support in different ways. Some teachers experience the FA toolbox as a source of inspiration but are less inclined to reflect with colleagues on how to improve the implementation of FA. The majority of the participants indicate that the toolbox raises awareness on a more purposive use of FA. These teachers work with colleagues as critical friends to discuss their vision on teaching and FA implementation. Lastly, a third group of teachers are already using many of the tools offered by the toolbox. In discussions with colleagues they mainly put forward suggestions, take up a coaching role and reflect on the contribution of their actions to the school policy.