Across the globe, social workers serve schools in a variety of capacities, providing services such as skills training; individual, group, and family counseling; crisis intervention; home visits; parent support and education; and advocacy for students, families, and school systems. To date, no synthesis of the literature exists examining tier 1 and tier 2 cross-national school-based social work interventions. Therefore, the purpose of this systematic review was twofold: (1) to identify tier 1 and tier 2 school-based interventions that involve social workers and (2) to examine the extent to which the interventions are efficacious with school-based youths. A computerized search with inclusion and exclusion criteria was conducted using several databases. Eighteen studies were included for the final sample in this review. Effect sizes were calculated for all outcomes to determine magnitude of treatment effect. Results indicated that most of the studies were conducted in the United States (n = 14) and half (n = 9) of the included interventions were tier 1. Many positive effect sizes were found. Interventions aimed to treat a variety of outcomes such as sexual health, aggression, self-esteem, school attendance, identity, and depression. More research is needed to determine the effectiveness of school-based social work worldwide.