Int J Exerc Sci 5(2) : 127-135, 2012. Observational methods have been a primary methodology used by physical educators for assessing teacher and student behaviors in school physical education (PE) classes for over 30 years. Observational instruments traditionally used in PE are economical, but are time intensive and complicated. Recently national PE recommendations have been promoted to encourage practitioners to achieve ≥ 50% of PE class in moderate to vigorous physical activity (MVPA). The purpose of this preliminary study was to develop, validate, and test the reliability of the Simple Activity Measurement (SAM) instrument for assessing MVPA during school PE classes. Students (N=36, representing grades 3-10) from a convenient sample of schools in San Antonio, TX were randomly selected to wear SUUNTOTM heart rate (HR) monitors during PE classes as an MVPA intensity measure, and the SAM instrument was used to observe 6 classes (N=281 students) for MVPA each minute using the SAM 0-10 scale. The SAM instrument was found to be a significant predictor for HR (r=0.555, r2=0.308, p\u3c0.05) using linear regression, and the intra class correlation coefficient to test reliability was found to be R=0.803, p\u3c0.05. Students averaged 88.5 % of class time spent in MVPA at the elementary level and only 36.5 % of class time spent in MVPA at the high school level. The results of this preliminary study indicate that the SAM has promise to provide school PE evaluators with an effective economical observation tool to document minutes of student MVPA in PE classes, and to promote accountability with national MVPA recommendations