This paper addresses the question: How do teachers and school leaders respond to high stakes testing of pupils transitioning from primary to secondary school? It explores how a new test, the Level 6 test, operated with regard to primary/secondary school relationships in England. It draws on an analysis of qualitative interviews with teachers and school leaders in 20 primary schools that took part in the test, 40 school leaders that chose not to and 20 secondary-school leaders. Theoretical work on social boundaries is utilised to develop an argument that this test and its results acted as a boundary signifier, crystallising many of the tensions between primary and secondary schools. These tensions included the role of accountability regimes in requiring schools to demonstrate progress; narrowing of the curriculum and teaching to the test; and the extent to which test results can provide a true representation of pupil attainment. We conclude by suggesting the potential of the boundary signifier concept in relation to other tests at the primary/secondary boundary and other key transition points in education, and consider whether such tests can act as an ideal boundary object, serving to help overcome, rather than cement, barriers between schools.