The paper presents the results of the analysis of the impact of various ventilation systems on indoor air quality and energy consumption, performed for a typical Polish elementary school that was built in 1970s. Simulations were made with the use of two computer codes: CONTAM W and ESP-r. A multizone model of the global capacity of 9464 m3 was performed. The model contained 17 classrooms and 10 additional rooms typical of such buildings. The simulations were made for the whole heating season, the breaks in teaching and twenty-four-hour variability of internal heat gains were taken into account. The goal of the tests was to find a compromising solution which would provide good indoor environment and economical energy consumption. Carbon dioxide concentration was used as the air quality indicator, the seasonal energy consumption of the whole building was selected as the energy efficiency indicator. The results clearly show that natural ventilation systems, traditionally applied in classrooms, cannot provide suitable conditions for learning. The CO2 concentration is very often more than 3000 ppm. Indoor air quality is better when classrooms are regularly ventilated during breaks by opening the windows. However, during periods of low outdoor temperature it causes large decrease in the indoor temperature below 10 oC. Better quality of the indoor air can be achieved by using mechanical ventilation, which makes it possible to keep CO2 concentration at the level of 1000 ppm.