This study examined the effects of single-sex and coeducational schooling on the gender gap in educational achievement to age 25. Data were drawn from the Christchurch Health and Development Study, a longitudinal study of a birth cohort of 1265 individuals born in 1977 in Christchurch, New Zealand. After adjustment for a series of covariates related to school choice, there were significant differences between single-sex and coeducational schools in the size and direction of the gender gap. At coeducational schools, there was a statistically significant gap favouring females, while at single-sex schools there was a non-significant gap favouring males. This pattern was apparent for educational achievement both at high school and in tertiary education. These results indicate that single-sex schooling may mitigate male disadvantages in educational achievement. (Contains 3 tables and 1 figure.)