Assessing and reporting students’ performance is an important part of every-day teaching. Doing so without focusing on marks but instead on supporting students in further learning by giving individual, process-oriented and task-related feedback to them is a central idea of formative assessment. Within a teacher-training-study as part of the research project Conditions and Consequences of Classroom Assessment (Co2CA), a special in-service teacher training has been developed to foster teachers’ knowledge about formative assessment when dealing with modelling tasks. A test on teachers’ pedagogical content knowledge (PCK-test) has been used to evaluate the trainings. Quantitative results point out that teachers having taken part in the trainings outperform teachers not having been trained in formative assessment if dealing with modelling tasks within the PCK-test.