Thesis: This thesis examines three issues in relation with diversity in the educational system. The first issue focuses on equal opportunities: how may we promote diversity in higher education when secondary education is segregated? We implemented a random experiment in order to evaluate the impact of a tutoring program that aims at encouraging underprivileged high school students to enroll into the most selective tracks of higher education. We show that such programs may face difficulties, especially when dealing with mid-level students, which may lead to a risk of increased inequality in underprivileged high schools. The second issue is the influence of the peer group: may modifications in the social environment of high school students have an effect on their subsequent education? We exploit a natural experiment occuring in France whereby a small fraction of 10th grade students are randomly allocated to their classes. We find that the most fragile students who end up in classes where they meet very few former classmates from 9th grade are more enclined to repeat the grade and have a lower probability of graduating high school. The third issue is segregation: to what extent to students experience different social environments in their classes or schools depending on their individual characteristics? We provide a detailed statistical analysis which allows us to measure quantitatively the extent of social and academic segregation between schools and between the classes within those schools.