Research on micro-institutional change typically characterizes agents as being involved in social conflicts to defend the institutional status quo or to mobilize against it. However, agency inside organizations can be precipitated by the need to resolve practice dilemmas in uncertain and ambiguous institutional contexts. The findings from a comparative case study of two public schools in the United States undergoing state-mandated reform demonstrate that, when agency is born of uncertainty and ambiguity rather than political conflict and struggle, micro-institutional change depends on the dynamics of agents’ peer learning in communities of practice. In some cases, agents’ communities are organized to effectively seed new ideas and generate social pressures needed to encourage the uptake of new ideas. In other cases, the pattern of agents’ interactions in communities results in persistent cognitive and social disorganization so that newly seeded ideas cannot take root or spread. I refer to the particular dynamics by which peer learning in communities of practice results in micro-institutional persistence or change as collaborative institutional agency.