The concentration of social groups in certain geographic areas and institutions is not a new phenomenon, but since the current migration crisis, one of the world's largest since the end of the Second World War, it has reached an unprecedented level. This poses important challenges to many education systems: the arrival of newcomers often leads to increasing enrolment of immigrant students in schools where there is already a significant socially disadvantaged or at-risk population. This situation has important implications in terms of scholastic performance, school equality, and social cohesion. This paper presents the case of Catalonia (Spain), which in the past few years has experienced a significant influx of foreign students, with the proportion of the student population rising from 2 per cent, 10 years ago, to 15 per cent today. During this time, middle-class families have withdrawn their children from lower-class schools -- a process known as 'social differentiation'. The combination of these two phenomena has gradually led to a situation of segregation in some institutions.