The practice of children learning together in mixed-sex schools developed over the course of the 20th century, at a time when the separation of the sexes was still the norm. However, it played a part in the democratisation of education. Mixed-sex education in state secondary schools in France began with the admittance of girls to boys’ institutions in the early 1920s. Using various scales of analysis, this article reveals how the co-education experiment began during the inter-war period. It seeks to show the circumstances that led central government to authorise the introduction of mixed classes, in particular the issue of the baccalaureate which necessitated regulatory changes. Maps showing the locations of co-educational establishments across France since the 1930s show that the co-educational process was well under way throughout the country.