Background: School-based interventions aimed at improving physical activity (PA), sedentary behaviour (SB) and sleep (i.e., 24-hour movement behaviours) are prevalent. However, the potential use of homework as an intervention method has been largely unexamined. Our objective was to assess the effectiveness of school-based health interventions which implement homework to improve 24-hour movement behaviours in primary school-aged children, whilst examining the moderating effects of study characteristics on intervention effectiveness. Methods: We searched CINAHL, PubMed, Scopus, SPORTDiscus, The Cochrane Library and Web of Science on 4th March 2024 using the following eligibility criteria: (1) participants were aged 5–12 years old; (2) school-based interventions that implemented homework specifically designed to improve one or more 24-hour movement behaviours; (3) randomised- or non-randomised controlled trials, or mixed methods studies where quantitative components included experimental or quasi-experimental data that could be clearly extracted; (4) device-based measured changes in individual or combined 24-hour movement behaviours, or their compositions, were reported. Data were extracted independently by two reviewers with study quality rated using the NIH quality assessment tool. Random-effects meta-analyses were processed to compute standardised mean difference (Hedges’ g), with subgroup analyses, and meta-regressions also conducted. Results: From 2,281 studies, 19 studies involving 13,160 participants were included for data extraction. Meta-analyses revealed significant favourable association for school-based interventions which implemented homework for sleep outcomes (g = 1.06, p < 0.0001) and SB (g = -0.20, p = 0.0034). No significant effects of the interventions compared to controls were found for PA. Meta-regressions revealed that longer intervention durations significantly improved PA (counts per minute; β = 0.14, p = 0.0241), with no significant effects found for sleep or SB. Subgroup analyses showed significant effects of intervention on SB in RCT’s in both theory-based and non-theory-based studies, though differences between subgroups were not statistically significant. Effects varied between pre- and post-implementations of 24-hour movement guidelines on SB, but these differences were also not statistically significant. Conclusion: These results highlight a significant gap in school-based interventions implementing homework targeting all 24-hour movement behaviours, emphasising the need for future interventions to focus on reducing SB and improving sleep for more beneficial outcomes. Registration: PROSPERO CRD42024518271.