In the field of school reform, traditional standard intervention studies are based on a reduction to a linear causal relationship in which teachers are regarded as entirely passive agents of policies, while policymakers and researchers create a grand design that is then applied or revised by teachers, resulting in more positive change for students. Activity-theoretical formative intervention studies, conversely, take as a basic principle the fact that teachers themselves will gain agency and take charge of the process. Thus, they will become change agents. Here, the focus is on triggering and sustaining an expansive transformation process led and owned by teachers. In Japanese schools, especially Japanese elementary schools, “Lesson Study” sessions held by teachers as a typical method of on-the-job training in schools represent traditional activity. However, “Lesson Study” that has become obsolete provides limited orientation in terms of the ways teaching methods and techniques are intended to achieve predefined discrete objectives and fragmentary contents of classroom lessons. This kind of in-school training is built on traditional and standardized technical notions of professional development. To go beyond such in-school training, activity-theoretical formative interventions in teacher learning and development attempts to engage teachers in collaborative interventions to facilitate teachers’ expansive learning that expands the object of their learning to changing the broader structure of an entire school as an activity system. This is the transformation of teacher learning and development in schools toward shared inquiry into desired objects, forms, and patterns of practice; thus, redesigning schools. In this presentation, I describe findings from a formative intervention study of teacher learning and development in the Tennoji National Teacher Training Elementary School that is attached to Osaka University of Education in Osaka City, Japan. In the school, teachers are engaged in shared inquiry into transformation of the school, facing new conditions that the Japanese Ministry of Education urgently requires them to address as fundamental reasons for the existence of national teacher training elementary, junior high, and high schools.