In France, the heterogeneity of the conditions of education and of school's contexts is viewed as one of the most important aspects of the crisis of schools. But is there only the transfer of the segregation of social circumstances onto school ? Or should one admit that school itself is involved with the definition of various conditions of education ? The following text presents a current synthesis of French research results regarding the question of effects of context, expecially effects of teachers and schools. Thereby the weight lies on the methodologistic problems that appear, on their nuances and on their complexity (the results themselves occur in a fixed context), and the necessity is indicated to differentiate between its effect and fairness. In addition, the contribution delves into the controversies that led to the research results and concludes by pointing to a question of political consequences : is it about the mobilisation of the teaching body, the control of parental demands, or even more fundamentally, the feasibility of schools to offer everybody a pedagogic environment of equal quality ?