A group of educators and administrators in an international school in Thailand collaborated for a year to devise and publish a policy document with aim to reform assessment practices of its faculty. The group’s beliefs derived from standards-based assessment leaders and its broad aim was to build a more coherent, accurate, and meaningful assessment system. Using Actor Network Theory as its theoretical perspective, this mixed-methods action research study explored the extent that the policy document changed the beliefs and practices of the faculty, the assessment materials within the system itself, and what other factors may also help account for any changes. The first finding is that the policy did lead to observable changes in practices of faculty traced in tests, quizzes, and the gradebooks that record assessments. A second finding is that the impact of the policy as an agent for change depends on the frequency that it is referenced.