Drawing on a research study of formative assessment practices in Irish schools, this paper traces the design, development and pilot of the Assessment for Learning Audit instrument (AfLAi)--a research tool for measuring teachers' understanding and deployment of formative teaching, learning and assessment practices. Underpinning the paper is an extensive body of international research connecting assessment for learning pedagogy with student self-regulation, mental health and well-being. Reflecting on the potential of the AfLAi as a research tool, an activity systems framework is advanced as a mechanism to engage researchers and teachers in meaningful site-based continuous professional development that supports teachers' interrogation of aggregated school data derived from their responses to the AfLAi. It is argued that by de-privatising classroom practice in this way and challenging teachers to examine self-reports of their understanding and use of assessment for learning pedagogy, the extent to which students are afforded opportunities to develop as self-regulating learners is laid bare. In turn, the teaching, learning and assessment conditions that serve to create and sustain self-regulation by students emerge. The paper is premised on a commitment to a biopsychosocial approach to mental health and to an inter-disciplinary, multi-lens, research agenda that will yield comprehensive, dynamic insights and understandings to inform future practice.