This paper contributes to debates on the benefits of single-sex and co-educational school environments by considering both single-sex versus co-educational schools and single-sex versus co-educational classes in co-educational schools. Two research studies provide the empirical basis for this discussion. One study was a 10-year-long investigation of two Australian secondary schools which had been single-sex schools and became co-educational secondary schools over a two-year period. The second study involved a two-year investigation in an English co-educational secondary school where single-sex mathematics classes were introduced for one cohort of pupils for five school terms, after which mixed-sex classes were reintroduced. Evidence relating to academic self-concept, pupil, parent and staff perceptions and academic achievement are discussed. Overall, the evidence suggests that co-educational environments create possible social/interaction disadvantages for girls, but that academic self-concept is not adversely affected by transferring from single-sex environments into mixed-sex ones.