The core theory of polycentric inspection is that when schools reach a certain quality threshold, they can further improve best through a coordinated, collaborative effort between clusters of schools and external agencies such as, for example, social services and training providers. The suggested role of an external agency with the respect and resources of the inspectorate is to provide stimulus and support to make the network effective. Using a bounded case study method, this research seeks to assess the potential of polycentric inspection as a tool for improving school effectiveness and outcomes. Evidence from this study suggests that this mode of evaluation has had a significant impact on improving schools, supporting teachers’ practice and, arguably, increasing student examination outcomes in the network examined. In consequence, it is suggested that these findings have wider implications for the changing conception of school evaluation and how improvement can be achieved in education.