The growing presence of children of immigrant families in the public school system in the bilingual region of Catalonia provides us with an opportunity to study how young multilingual and multicultural speakers construct their social competencies and their identity within the specific context of a gate-keeping social institution such as the school. The study reported in this paper approaches language learning as a process of socialisation that involves not only learning how to make sense of linguistic signs but also learning how to enact different social roles in particular institutions. The analysis focuses on the interactional profiles of two immigrant students in two types of communicative activities that are representative of the school context: responding to questions from an adult and cooperating with a peer in the resolution of a learning task. By shifting the focus of analysis from a decontextualised notion of communicative competence to the notion of "institutionally affiliated communicative competence" and concentrating on issues such as the (1) the relationship between knowledge and participation, (2) language choice inside and outside school and (3) definitions of correctness in language use, the study reveals how the two students construct a highly "affiliated" identity. (Contains 3 figures, 2 graphics and 2 notes.)