Our paper presents a simplified model of carbon dioxide concentrations in classrooms equipped with stack ventilation systems, based on experimental research. The test was conducted in six classrooms in the building of the Faculty of Civil and Environmental Engineering of Białystok University of Technology in northeastern Poland. The research included both classrooms that were not mechanically ventilated and classrooms that were ventilated by opening the windows during breaks between classes. In all classrooms, a linear increase in the concentration of carbon dioxide during the classes was observed. The increase of the concentration of carbon dioxide in spaces of this type depends mostly on the volume of and the number of persons present in the space.