Construction et évaluation de l'effet établissement : le travail des collèges Mr Olivier Cousin Résumé A survey was conducted among the pupils of twelve junior high schools from the second to the fourth year. Selection and orientation mecanisms were analysed in each phase of the school system, with the help of a multiva-riated regression method. Though some variable have a strong predictive capacity, concerning school success (social origin, age, sex), pupils selection differs according to the schools. So colleges produce (partly) school failure. Though there is a strong correlation between the social composition of a school and pupils orientation, some results are significantly different. This is due to a school effect : the selectivity of a school is not only the result of its social composition, it is also the product of the actions and behaviours of the actors (leaders, teaching staff). Social cohesion differs according to the schools, the least selective are those which have collective goals and do not separate educationals tasks and pedagogical acts.