This study examined how immigrant mothers’ ethnic socialization practices influence the multicultural identity and school adjustment of their adolescent children. Using data from the 2021 National Survey of Multicultural Families in Korea, this study analyzed a matched sample of 8,296 mother–child pairs and conducted ordinary least squares (OLS) regression, including interaction terms for children’s academic achievement and experiences of discrimination. The results show that cultural transmission socialization by immigrant mothers did not exhibit a significant direct effect on school adjustment; however, it had conditional effects on multicultural identity, depending on children’s academic achievement and discrimination experiences. In contrast, preparation-for-bias socialization was more closely associated with school adjustment than with multicultural identity, and its influence varied by children’s academic performance, serving as a protective factor for those with lower achievement but undermining adjustment for higher-achieving adolescents. These findings suggest that immigrant mothers’ ethnic socialization exerts context-dependent effects shaped by children’s academic achievement and social experiences. This study contributes to extending ethnic socialization research in the Korean context and offers practical implications for parent education and policy support for immigrant families.