Indoor air quality (IAQ) is very important for children health and well-being, since children are particularly vulnerable and sensitive on presence of air pollutants. This study was performed in two naturally ventilated schools located in the same municipality. First school is located in urban area, at residential - industrial site, while the other school is situated in rural area. School buildings were chosen based on their urban environment features. The measurements were carried out in heating as well as in non-heating period in duration of five consecutive working days. The objective of the study was to analyze IAQ in the classrooms with special emphasis on levels and diurnal variations of particulate matter (PM10 and PM2.5), carbon dioxide (CO2) and nitrogen dioxide (NO2) in occupied and unoccupied school classrooms. In this paper, the CO2 concentrations were measured at both indoor and outdoor environments. Concentrations of CO2 higher than 1000 ppm were regularly detected in the classrooms during teaching hours. Indoor concentrations of PM10 were not exceeded the guideline, daily average, value of 50 μg/m3. Concentrations of PM2.5 were exceeded the guideline, daily average, value of 25 μg/m3 in both school during heating period. Concentrations NO2 were not exceeded the guideline value of 200 μg/m3. Ventilation rates were calculated and compared with the prescribed limits. In both occupied and unoccupied periods high correlation between CO2 and PM concentrations was determined. [Projekat Ministarstva nauke Republike Srbije, br. III42008]