This meta-analysis study synthesized the intervention effects of Check-in/Check-out (CICO), the most common Tier 2 behavior intervention used in schools. Systematic review procedures were employed to examine six CICO studies that were evaluated using a group research design and that involved 146 students. Studies were coded to examine their characteristics as well as quality indicators concerning their design features. The results indicated that in group design studies, the overall effect size of CICO across student outcomes (problem behavior, appropriate behavior, social skills, and academic performance) was medium (g = 0.42). No differences in effects were found for outcome domains, Tier 1 fidelity, and implementation fidelity, but grade level was associated with statistically significant differences in effect sizes (Q = 3.95, p < 0.05). Elementary school students (g = 0.70) showed greater improvements than middle school students (g = 0.27). Further research is needed to examine the intervention features impacting the effects of CICO.