Schools are the only platforms that have the ability to regularly reach the majority of school-age children and adolescents across the globe. Although at least 102 countries have school health services, there is no rigorous, evidence-based guidance on which school health services are effective and which services should be implemented in schools. To work towards the development of such guidance and investigate the effectiveness of school health services for improving the health of school-age children and adolescents, a systematic review of systematic reviews was conducted. PubMed, Web of Science, ERIC, PsycINFO, and the Cochrane Library were systematically searched through June 2018. Systematic reviews of controlled intervention studies that evaluated school-based or school-linked health services delivered by a health provider were included. Review quality was assessed using a modified Ballard and Montgomery four-item checklist. 1654 references were screened and 20 systematic reviews containing 270 primary studies were adjudged to be eligible and were assessed. The majority of systematic reviews assessed interventions related to health education, counseling, and other preventive interventions, most of which were research interventions rather than routinely-delivered school health services. No systematic review evaluated the effectiveness of a comprehensive or multi-component school health services intervention, though this may have been at least partly related to the requirement that the reviews were of controlled studies. Systematic reviews of specific interventions concluded that there was evidence for effectiveness of interventions that addressed autism, depression, anxiety, obesity, dental caries, visual acuity, asthma, and sleep. No systematic reviews were found that addressed common services, such as hearing screening, or potentially important specific interventions, such as substance use prevention interventions. The strongest evidence from the reviews identified supports implementation of anxiety prevention programs, indicated asthma education, and vision screening with provision of free spectacles. Results suggest that certain interventions can be effective in improving child and adolescent health outcomes, and thus should be considered for integration into school health programs. Evidence-based recommendations on which services should be available in schools, who should deliver them, and how should they be delivered, will require additional systematic reviews. These must not only assess the effectiveness of specific health interventions for which there is no recent review, but should also assess routine, multi-component school health services and the characteristics that make them effective, with special attention to content, quality, intensity, method of delivery, and cost. The gaps in the systematic review literature identified by this overview will inform the commissioning of new systematic reviews by the World Health Organization to feed into evidence-based global recommendations.