This study was conducted to validate the System for Observing Fitness Instruction Time (SOFIT) for measuring physical activity levels of high-school students. Thirty-five students (21 girls and 14 boys from grades 9-12) completed a standardized protocol including lying, sitting, standing, walking, running, curl-ups, and push-ups. Heart rates and Energy Expenditure, that is, oxygen uptake, served as concurrent validity criteria. Results indicate that SOFIT discriminates accurately among high-school students' sedentary behaviors (i.e., lying down, sitting, standing) and moderate to vigorous physical activity behavior and is recommended for use in research and assessment of physical activity levels in physical education classes for this age group. Implications for use of SOFIT by both researchers and teachers in physical education are described, as well.