Children spend on average 7-10 per weekday at school, that’s why the indoor air quality in the classrooms plays a key role in the assessment of the effects of their personal exposure to the air quality. Many scientific articles indicate the substantial influence of carbon dioxide (CO2) levels and overall air quality within educational environments on the well-being and cognitive performance of children. This article presents the case study of the classroom in the primary school in Cracow with very unfavorable indoor air quality caused by the usage pattern. In the classroom, there is a natural ventilation system, still the most common in the Polish existing buildings. The very high level of CO2 exceeding the standard requirements connected with low ventilation efficiency effects in harmful indoor conditions. Based on the measurements conducted in the classroom during the lessons with the users in and taking into account formal requirements authors assessed the quality of indoor air. The main reason for those unfavorable conditions is an inefficient natural ventilation system. This paper is also supposed to answer the question of whether temporary opening windows can assure proper concentration of CO2 in a standard classroom and, if not, what would be the optimal ventilation rate. In the next step, this optimal minimum required ventilation rate for the classroom was calculated. It could be used as a design assumption in the selection of a ventilation system.