Since the introduction of the Prevent duty across the UK, schools have had to balance the need to fulfil their responsibilities under the duty -- often understood to include monitoring and surveillance -- with their ultimate purpose to educate their students. This positions teachers within a particular set of tensions about their own beliefs about education, their values, and their roles and relationships with young people and communities. This article draws on interviews with classroom teachers and members of school leadership teams from 10 schools, in order to compare how teachers have understood and responded to those tensions. The article will focus on the various ways in which teachers frame the policy, and the ways in which they exercise agency in their responses. Drawing on an ecological approach to theorising teacher agency our data reveals how teachers develop different responses to anti-extremism policy depending on their role; their school contexts; and their own beliefs. Whilst in some important regards the statutory Prevent duty has 'closed down' some options, nevertheless teachers exercise agency to interpret and enact policy and, when translating the policy into a curriculum context, also make 'leaps' of interpretation as concepts such as fundamental British values are turned into lessons.