While a range of sociodemographic characteristics are associated with a greater risk of peer rejection at school, it is currently unclear how key theoretical frameworks explaining rejection apply to such characteristics. This study examines how migration background, gender, household income, parental education and cognitive ability are linked to peer rejection. Building on person-group dissimilarity and social identity theory, the study assesses the moderating role of classroom composition and the extent to which students reject classmates who differ to themselves (i.e., outgroup derogation). Data is drawn from a nationally representative sample of 4215 Swedish eighth grade students (Mage = 14.7, SDage = 0.39; 67% of Swedish origin; 51% girls) in 201 classes. While rejection based on migration background, gender, household income and cognitive ability was moderated by the school-class composition, only the rejection of immigrant background students, boys and girls was related to outgroup derogation. Furthermore, Swedish origin students’ outgroup derogation increased as the share of immigrant background students decreased. Addressing social inequalities in rejection may require different strategies depending on sociodemographic characteristic.