OBJECTIVE: To quantify the physical activity (PA) of students and describe the school environment surrounding PA. MATERIAL AND METHODS: Between November 2005 and March 2006, in Mexico City, we conducted quantitative and qualitative observations to describe the PA and the school context. RESULTS: Recess and physical education class (PE) were the only opportunities to participate in PA. PE occurred one time per week with a duration of 39.8+/-10.6 minutes which is less than national and international recommendations. Students participated in moderate-to-vigorous PA 29.2+/-17.8% of PE. The dynamics of PE did not promote the inclusion of all students or PA. During recess there was overcrowding of the school patio and no equipment for PA or organization of PA. DISCUSSION: The PA of students in public schools in Mexico City can be improved by increasing the quantity and quality of PE and increasing opportunities for activity during recess.