One reason reform does not dramatically change public schools is because instructional practices are highly institutionalized. This article advances a theory for how teacher agency can both change and maintain institutionalized instructional practices in schools. Based on findings from one U.S. urban public school undergoing state-mandated reform, I assert that three mechanisms drive a particular form of teacher agency. Whether these three mechanisms change or maintain institutionalized instructional practices depends on a set of counterbalancing forces that determine how much innovation versus socialization exists in peer learning; how much cohesion versus diversity is involved in community interactions; and how much cognitive and normative divergence versus convergence characterizes teachers’ shared understandings, aims, and practices. The theory provides a generalizable framework for the activities that help teachers drive outcomes in their schools and for the cognitive and social conditions that may be more likely to result in the effective implementation of reform.