Purpose: To examine the reliability of age-adapted submaximal Yo-Yo (Yo-Yo_submax) intermittent tests in untrained schoolchildren aged 9–16 years (n = 139; 72 boys and 67 girls) and within children with high and low percentage of body fat (%BF). Methods: Yo-Yo intermittent recovery level 1 children’s (YYIR1C), Yo-Yo intermittent endurance level 1 (YYIE1), and Yo-Yo intermittent endurance level 2 (YYIE2) tests were performed 7 days apart by 9- to 11-, 12- to 13-, and 14- to 16-year-old children, respectively. Reliability was tested for Yo-Yo_submax heart rate (HR_submax), peak HR, and maximal distance. Results: HR_submax typical errors of measurement (TEM) in YYIR1C, YYIE1, and YYIE2 were 2.2% (1.7%–2.9%), 2.4% (1.9%–3.3%), 1.9% (1.6%–2.5%) and 2.4% (1.9%–3.3%), 2.4% (1.9%–3.2%), 1.9% (1.5%–2.4%) for girls and boys, respectively. HR_submax intraclass correlation coefficient values were good to excellent (.62–.87) in all age groups and in schoolchildren of different %BF. TEM for HR_submax ranged from 2.1% to 2.3% in high and low %BF groups. Maximal distance intraclass correlation coefficients were excellent and TEM values ranged from 11% to 12% in both %BF groups. HR_submax was moderately to largely associated (r = −.46 to −.64; P < .002) with Yo-Yo maximal distance across the age groups. Conclusion: Yo-Yo_submax tests are a reliable tool providing useful and sustainable aerobic performance testing in physical education, irrespective of individual %BF.