Whilst we admit to recognising the contextualised character of pupils' learning, the no less essential influence of the parameters of the school or social context remains a relatively unemplored field. Of course, the mobility of school effectiveness amply demonstrates the influence of the school of class attended by the pupils on their progression. But in the search for general factors of effectiveness, this trend has had few questions asked of it concerning the relationship with the characteristics of the pupil public. However, all work carried out on the impact of the forms of grouping of the pupils shows that both the average level of the group and the social composition (known in the Anglo-Saxon world as "school mix") affect the average progress, and to the same extent, the inequalities between pupils. This influence goes by the modulations which are created by the form of grouping by the quantity and quality of instruction delivered in the classroom on the one hand, and on the other with regard to the attitudes and behaviour of the pupils assigned to the different groups.