Many schools worldwide closed to contain the spread of the COVID-19 virus. However, the consequences of school closures for the school adjustment of adolescents from different ethnic and SES backgrounds remain unclear. This study examined how school adjustment changed before, during, and after school closure across adolescents from different ethnic and SES backgrounds; and which factors in home and school contexts served as resources. Early adolescents (N = 124, Mage = 12.86, 58.8% boys) from different ethnic and SES backgrounds were repeatedly assessed 1 week before (March 2020), during (June 2020), and 1 year after (February 2021) the first school closure in Belgium. The results revealed that school closure augmented ethnicity- and SES-based inequalities in school adjustment. Moreover, factors in the school context—and not the home context—served as resources. Specifically, the quality of online instruction and teacher-pupil relationships buffered against reduced school adjustment during school closure, particularly among youth from ethnic minority and lower SES backgrounds. The findings corroborate unequal school adjustment consequences of school closures, but also highlight the role of teachers to buffer against them. The study design, hypotheses, and analyses were preregistered in the following link: https://osf.io/6ygcu/?view_only=c77cfb46028447bdb7844cd2c76237aa.