This article highlights how school can work as a "pressure cooker valve", contributing to a "controlled management" of processes of exclusion. Schools, sometimes without realizing it, simultaneously promote some practices that lead to greater inclusion and support, and others that effectively result in the exclusion of minorities. This reasoning is illustrated, in the Portuguese context, with data collected in situations that demonstrate not only the different cultural, social and economic backgrounds of the students, but also the homogenizing rigidity and exclusionary effects of the curricula and teachers' practices. In particular, this paper points to the negative and exclusionary impact of a limited range of teaching and learning opportunities. We affirm the growing social and cultural heterogeneity of students attending Portuguese schools, but argue that they are educated in a system, and by teachers, that usually promote a dominant culture, established as the only one that it is important to acquire. We propose that a true dialogue can only exist if teachers and students soften the barriers between their cultures in order to interact. (Contains 4 notes.)