Argued to "raise boys' grades" and "boost boys' academic achievement", single-sex classes in coeducation schools is one strategy among a plethora aimed at raising standards. This paper explores the experiences of teachers in one coeducation post-primary school that sought to raise academic performance, particularly among boys, and to improve classroom behaviour by introducing single-sex classes. Funded by a local Education and Library Board the evaluation took place almost four years after the strategy was introduced and in a climate of increasing teacher concern with the effectiveness of the strategy. The methods involved a questionnaire survey to all teachers in the school and one-to-one and small group interviews with teachers responsible for teaching single-sex classes. Contrary to its stated aims, the majority of teachers believe that, since the introduction of single-sex classes, academic performance and classroom behaviour have deteriorated. Findings are discussed in terms of teachers' perceptions of the strategy and the importance organizational and contextual factors have on shaping teachers' attitudes to new initiatives. (Contains 4 tables.)