English secondary schools operate within a performance management system, which includes league tables reporting school performance across a number of indicators. This article reports the results of an interview-based study, showing that head teachers care about their school's place in the league tables, and that they believe this system affects behaviour. The effects they identify include some unintended consequences, not necessarily related to improved overall school performance, including focusing on borderline students who can boost a pivotal indicator: the number of students gaining five A*-Cs at GCSE. This behaviour reflects, in part, the dual role played by headteachers: they are both educationalists (serving the interests of all pupils); and school marketers, concerned with promoting the school to existing and prospective parents. The behaviour is also consistent with economic theory, which predicts a focus on that which is measured, potentially at the expense of that which is important, in sectors characterised by incomplete measurement, by multiple stakeholders and containing workers with diverse objectives. We conclude that, given that performance indicators do affect behaviour, it is important to minimise unintended consequences, and we suggest how the use of value-added indicators of student performance may improve both what gets measured and what gets done in secondary schools.