Background: The structure provided by school settings even with no specific obesity-intervention may prevent weight gain. This meta-analytic study considered this premise by examining weight outcomes from control groups in published randomized controlled trials of school-year obesity-related interventions conducted in-school and out-of-school. Methods: A systematic review and random-effects meta-analysis were conducted in accordance with Preferred Reporting Items of Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Databases were systematically searched and resulted in 1976 unique citations, with 14 retained for analysis. Analyses examined the change in weight outcomes among control group participants. Results: For studies (N = 6) reporting body mass index (BMI) (kg/m[superscript 2]) the overall standardized mean difference (SMD) from pre- to post-intervention was 0.085 (raw units 0.278 kg/m[superscript 2]); for studies (N = 9) reporting zBMI, the SMD was 0.022 (0.020 z-scores), for studies (N = 2) reporting waist circumference (cm), the SMD was 0.149 (1.609 cm); for studies (N = 2) reporting BMI percentile, the SMD was 0.064 (0.985 percentiles); and for studies (N = 1) reporting percent body fat, the SMD was 0.031 (0.30 percentage). Conclusions: Children assigned to control conditions (as part of school-based obesity-related interventions) experience, on average, minimal changes in weight outcomes during the school year. Therefore, routine practices of schools may protect against unhealthy weight gains.