Objectives: This study systematically evaluated the effectiveness of school-based interventions in reducing internet gaming disorder (IGD) severity among adolescents. Methods: A systematic review and meta-analysis were conducted, searching international databases (MEDLINE, Embase, PsycINFO, Cochrane CENTRAL, CINAHL), and Korean databases (DBpia, KCI, RISS, KIST) for studies published before May 2024. Nine quasi-experimental studies with 764 participants were included. Effect sizes were calculated using a random-effects model (Hedges’s g) with 95% confidence intervals (CIs) and heterogeneity statistics (I2). Subgroup analyses examined intervention type, duration, session, and participant characteristics. Risk of bias was assessed using the ROBINS-I tool. Results: Meta-analysis revealed a significant reduction in IGD severity (Hedges’s g=-1.171, 95% CI [-1.673, -0.669]) with high heterogeneity (I2=87.48%). Educational interventions were more effective than physical activity-based programs (Hedges’s g=-1.289 vs. -0.775). Interventions led by psychologists showed greater effectiveness than those led by teachers (Hedges’s g=-1.581 vs. -0.884). Short-term interventions (<3 months) with at least 10 sessions and durations of ≥1 hour demonstrated the highest efficacy. The higher the boy-to-girl ratio and primary school students responded more positively than other groups. Heterogeneity was attributed to variability in study designs and participants demographics. Conclusion: School-based interventions significantly reduce IGD severity among adolescents, particularly when tailored to developmental and demographic needs. Teacher-led and psychologist-led interventions were the most effective, highlighting the importance of design and delivery. Future studies should adopt standardized evaluation tools, include randomized controlled trials, and implement long-term follow-ups to strengthen evidence. Additionally, incorporating family support and tailored IGD strategies may enhance interventions successful in school settings. This study systematically evaluated the effectiveness of school-based interventions in reducing internet gaming disorder (IGD) severity among adolescents.