The K-12 education system is the largest public enterprise in the United States. Public K-12 schools employ approximately three million staff and enroll more than fifty million students. Public schools have an expenditure that includes funds for salaries, benefits, transportation, materials and energy use. Of these expenditures, energy use is often targeted for reduction, which sometimes results in reduced ventilation. While saving energy is desirable, it must be achieved without compromising student health. Lack of proper ventilation can result in a decrease of students’ attention and can cause health-related issues. This paper evaluates how the type of heating, ventilation, and air conditioning (HVAC) systems installed in schools and HVAC operation conditions affect temporal and spatial distributions of carbon dioxide (CO2) concentration in classrooms.