This article reviews school effectiveness theory, concentrating on the unidimensionality of the school effect concept, and focuses on differential school effectiveness, by which is meant the capacity of the school to be effective with different groups of pupils. It presents findings from research exploring the associations between sex, social class, and school attended with Cypriot primary pupils progress in Mathematics. There was no evidence of significant differential effectiveness in relation to sex and social class, the gap between boys and girls, and between different social classes increased in all schools, reflecting the national picture. Implications for school self-evaluation are discussed.