The spatial, institutional and social configurations of school supply and demand are crucial aspects in understanding the various mechanisms of production and reproduction of socio-spatial inequalities in education. The same policy instruments may have different effects depending on the characteristics of local education markets and the dynamics of supply and demand in each of them. This article investigates how a policy designed to reduce school segregation in the city of Barcelona resulted in uneven effects in different catchment areas of the city, which are socially and educationally diverse. By comparing the effects of the same policy strategy in different territories, the article identifies four mechanisms that mediate and occasionally prevent the effectiveness of the policy instruments or their implementation procedures. Identifying these mechanisms is an important and necessary task when reviewing the existing policy design and adapting it to the particularities of local education markets.