The purpose of this study was to investigate the association between activity levels assessed by a uniaxial accelerometer and metabolic equivalents (METs) during walking and running in school students. The participants were elementary (10 boys, 10.3 ± 0.8 years), middle (10 boys, 13.0 ± 0.9 years) and high school students (10 boys, 15.7 ± 0.7 years) who performed treadmill walking (1, 3, and 5 km/h respectively) and running (5, 7, and 9 km/h respectively) trials. Activity levels were assessed using a uniaxial accelerometer (Lifecorder EX; Suzuken Co. Ltd., Nagoya, Japan). Energy expenditure was measured by indirect calorimetry. METs and activity levels indicated by Lifecorder were highly correlated in elementary (y = 0.0432x² + 0.0914x + 1.786, r = 0.883, p < 0.05), middle (y = 0.0781x² - 0.0262x + 2.1098, r = 0.913, p < 0.05), and high (y = 0.0516x² + 0.1863x + 1.7812, r = 0.876, p < 0.05) school students. We confirmed that higher accuracy showed LC5, LC4, and LC4 for elementary, middle, and high school students, respectively. In conclusion, this study suggests that Lifecorder intensity for ≥3 METs physical activity indicate ≥5, ≥4, and ≥4 for elementary, middle, and high school students, respectively.