Background. Schools are sites of dating and relationship violence (DRV) and of gender-based violence (GBV) victimization and perpetration. School-based interventions can reach a broad range of students, targeting both individual and group processes that may underpin DRV and GBV. Considering DRV and GBV jointly is important because of their shared etiologies. Comparing the effectiveness of interventions using network meta-analysis (NMA) can support decision-making on optimal resource use. Objectives. To evaluate the comparative effectiveness of school-based interventions for children aged 5 to 18 years on DRV and GBV victimization, perpetration, and related mediators. Search Methods. We searched 21 databases in July 2020 and June 2021, alongside extensive supplementary search methods, including gray literature searches, forward and backward citation chasing, and searches on first and last author names. Selection Criteria. We included randomized-controlled trials of interventions for children of compulsory school age implemented within the school setting, and either partially or wholly aimed at changing DRV or GBV outcomes. Data Collection and Analysis. Pairwise meta-analyses using random-effects robust variance estimation considered intervention effectiveness on DRV and GBV victimization and perpetration using odds ratios, and on mediators (e.g., knowledge and attitudes) using standardized mean differences. Effects were divided into short-term (< 12 months postbaseline) and long-term (≥ 12 months postbaseline). NMAs on victimization and perpetration outcomes compared interventions categorized by breadth of mechanism and complexity of delivery and implementation. Meta-regression tested sensitivity to percentage of girls in the trial sample and country context. Main Results. Our analysis included 68 trials. Evidence was stronger overall for effects on DRV than for GBV, with significant long-term impacts on DRV victimization (odds ratio [OR] = 0.82; 95% confidence interval [CI] = 0.68, 0.99) and DRV perpetration (OR = 0.78; 95% CI = 0.64, 0.94). Knowledge and attitudinal effects were predominantly short-term (e.g., for DRV-related violence acceptance, d = 0.16; 95% CI = 0.08, 0.24). NMAs did not suggest the superiority of any intervention type; however, most analyses for GBV outcomes were inconsistent. A higher proportion of girls in the sample was associated with increased effectiveness on long-term victimization outcomes. Author's Conclusions. Evidence is stronger for DRV than for GBV, despite considerable heterogeneity. Certainty of findings was low or very low overall. Public Health Implications. Violence reductions may require more than 1 school year to become apparent. More extensive interventions may not be more effective. A possible reason for stronger effectiveness for DRV is that whereas GBV is ingrained in school cultures and practices, DRV is potentially more open to change via addressing individual knowledge and attitudes. (Am J Public Health. 2023;113(3):320-330. https://doi.org/10.2105/10.2105/AJPH.2022.307153).