In light of recent emphasis on K-12 scientific modeling, it is important to understand how students’ models and beliefs about modeling shape shared classroom practices, and how, in turn, shared classroom practices influence individual students’ practices. We use co-operative action to consider the ways in which sedimented practices and artifacts become part of the substrate for students’ later actions ). Lemke :273–290, 2000) and Goodwin describe and provide illustrative examples of the accumulative nature of transformation of materials and practices. However, (...) these examples range in scale from minutes to hours, and there is less guidance about applying these perspectives to consider the transformation of practices longitudinally. In this study, we find that co-operative action is a powerful framework for analyzing classroom practices. We show that new practices rippled through the classroom along three dimensions: immediate transformation, longitudinal reuse and transformation, and transformation through interaction with other practices. These findings confirm that students’ models and practices become part of the substrate for later enactments of practice, and that these practices develop co-operatively and accumulatively within the classroom. Thus, co-operative action provides a unique lens on student progress that complements learning progressions by considering the social dimension of the development of practices. We propose that the extent to which students’ practices are able to sediment into the substrate of shared classroom practices is an important indicator of the health of the classroom as a scientific community. (shrink)