This text, pressed on a quantitative and qualitative analysis, tries to describe and to understand in what middle schools differ by their optional teachings. The results establish a differentiation according to the social composition of middle schools. The more the middle school is socially favored, the more the diversification of teachings is low and specialized in the excellence. The modest middle schools are more diversified and more appeal to devices with excellence, professional and integration. Besides, the "educational" disparities between middle schools are supported by justifications, conceived according to different conception of the common good by the management personnel.