Background/Objectives: Physical activity and sedentary behavior are key indicators of youth health, and questionnaires offer a cost-effective method for data collection. This systematic review synthesized evidence on the criterion and convergent validity of self-report questionnaires measuring physical activity and sedentary behavior in children and adolescents aged 5–18 years in school settings. Methods: Searches were conducted in SPORTDiscus, Medline, Scopus, ERIC, Academic Search Complete, Education Source, and the Cochrane Library with no date limits. Included studies compared questionnaire results with established measures and were assessed using the consensus-based standards for the selection of health measurement instruments (COSMIN) checklist. Results: Twenty-two studies covering 16 questionnaires were included; additional outcomes included test–retest reliability, content and construct validity, and psychometric evaluation. Validity correlations ranged from very weak to strong (r = 0.11–0.80; ρ = 0.20–0.87), with Bland–Altman agreement (ρ = 0.56–0.70) and mean absolute percentage error up to 199.6%. Conclusions: Eight questionnaires showed acceptable validity and reliability, though research is needed to strengthen evidence on measurement accuracy.