In the social sciences, the term “peer effects” has been widely used to describe the various ways in which individual behaviors and attitudes can be influenced by friends, acquaintances, and the wider social environment. Due to the crucial role of social interactions within the school context, the role of peers in shaping academic outcomes has been under scrutiny for decades. Following seminal work by Manski, we distinguish between three different components of peer influence: endogenous (where the behavior of an individual varies in accordance with the behavior of the peer group), exogenous (where the behavior of an individual varies with the characteristics of the members of the peer group), and correlated (where the behavior of individuals is shaped by shared environmental or institutional factors). By estimating a simultaneous autoregressive model, we assess the relative strength of these three forms of peer influence in relation to secondary school exam results in a large sample of Italian school-leavers. One limitation is that we are only able to observe peer influence within the classroom, while another is that the study is confined to a specific moment in time, which comes quite late in young people’s educational trajectories. The results confirm that peer processes play an important role in the reproduction of social inequalities, against the backdrop of institutional criteria for the selection of students into schools and classes. These factors therefore demand the sustained attention of educational administrators and policymakers.