In September 1977, all of France's middle schools implemented the so-called "collège unique" reform in the seventh year, i.e. the elimination of tracking (or streaming) and the mixing of all pupils and teachers for the first year of secondary education. This article examines the genesis and implementation of this reform from the point of view of the budgetary issues it raised and which shaped it. Drawing on the political economy of education, the article sheds a new light on the introduction of comprehensive education, of which different variants can be observed in Western countries in the 1960s and 1970s. Indeed, far from being anecdotal, budgetary issues very concretely shaped the practical conditions of implementation of this educational reform, putting political and economic considerations ahead of any concern for pedagogical coherence.