One of the focuses of language revitalisation policies is to incorporate minority languages into education. Evaluation of new language-of-instruction models is usually based on the increase of minority language speakers. However, it is also important from an educational perspective to study the possible relationship between performance and significant variables such as economic and cultural status (ESCS) or language background. In the context of the Basque Autonomous Community, where three different language-of-instruction models coexist, we analyse data from a diagnostic assessment of mathematics competence administered to 16,993 fourth-grade students. Using multilevel models, the results show an association between linguistic groups and ESCS, and that the relationship is not strong enough to explain performance differences among groups. The research demonstrates the importance of taking linguistic and socioeconomic background into account when comparing academic achievement among language-of-instruction models.