The influence that co-ethnic relationships and co-ethnic networks have on the educational attainment of immigrant students is a topic that has been researched very much within the sociology of migrations. During the last years, the concept of social capital has been privileged in the analysis of this topic, highlighting fundamentally the positive effect of such relationships and networks over the educational attainment of immigrants’ pupils. Nevertheless, the opposite outcome is shown by a second sort of literature that focuses on the analysis of a particular form of this kind of relationship: the one that occurs inside the peer groups. Researches included within this second sort of literature usually highlight the negative effects of these co-ethnic relationships on immigrant students. This article makes a dialectic contrast between both tendencies in the literature, which allows us to face the analysis of the association between co-ethnic relationships and the educational attainment of immigrant pupils from a critical perspective. As a result, this paper underlines the ambivalence of the ethnic factor that is implicit in this kind of relationships.