The research presents a study of ethnic homophily, i.e. the tendency of agents to be connected to similar others, in the school context. It uses two observations of the complete social network of a cohort of Swedish students (n=115) in a secondary school. The study analyses the scope of the selective attachment and the student’s possible motivations to seek similarity, as well as its interrelation to homophily induced by structural organization or foci. It focuses in the interrelation between the individual motivation to seek similarity and how the context of opportunity is organized. Sociometric data was collected during a school year and the model is based in crosscuts of the friendship network. Homophily in respect to ethnic origin was found in the first time point, with students making connections within the in-group. The effect of ethnic homophily seems to decrease between the time points, with evidence of an effect of structural constraints that gained importance over time. Results suggest that organisational divisions in classrooms play a decisive role in the pattern of friendship connections in the school