The aim of this study is to analyze the nature of multiethnic academic interactions in relation to theories of cultural capital and boundary-work. More precisely, it considers to what extent school structure is related to the cultural capital of students from different ethnic backgrounds and explores its relationship to Intergroup Contact Theory and identity. Methods include documentary analysis, participant observation, interviews, and focus groups conducted from an ethnographic perspective between 2015 and 2016. Based on data collected in a Basque school attended by a high proportion of immigrant students, intraethnic and interethnic student–student and student–teacher relationships, and inequalities within these, are analyzed. Results indicate that the distribution of students in different classes tended to be ethnically marked, as most immigrant students chose to attend classes that were taught mostly in Spanish, whereas most autochthonous students were enrolled in classes with a high Basque instruction. The study considers the effects of students’ language choices and concludes that Basque has implications for the theories of identity, cultural capital, and boundary-work, as learning Basque is an academic and implicit rule in Basque education and society.