Cultural-historical activity theory (CHAT) has been used in several development studies, but few have used it for the purposes of formative assessment. This article reports from a study where the activity system was used to conduct a historical and actual-empirical analysis of the assessment culture at a purposively selected school. The article focuses on the theoretical and methodological aspects of interventionist research, based on a previously conducted empirical study. Mirroring was used as an interventionist technique in workshops with the researcher, using comprehensive data from the empirical study. The discussions in this article show how contradictions are the driving force of development for formative assessment. It suggests that formative interventions can bridge the gap between perceptions and practices of assessment, which seems to be a prerequisite for aligning assessment, teaching, learning, research, and practice. As such, this article provides new knowledge on how CHAT can inform and enhance formative assessment.