There are more than 200 thousand pupils in Latvia. Most of them are still learning in non-renovated classrooms without proper mechanical ventilation. The classrooms are often ventilated only during the breaks by opening windows. This can lead to increased CO2 levels and reduced mental performance. To test how CO2 concentration in classrooms influences student attention level and their ability to perform mental tasks the students had to complete a short test at the start and the end of the class. At the same time CO2 concentration, temperature and relative humidity were logged. Also, an anonymous survey on how the pupils fell regarding the overall IEQ in the classroom, their thermal sensation, are they fatigue, have difficulty concentrating and if they have headaches during the lesson was performed. The measurements were performed in a Secondary School in Daugavpils, Latvia. The analysis of results shows that existing 10-minute breaks are not enough to fully ventilate the classroom, and they must be increased to at least 15 minutes. At the same time, 30-minute breaks can be reduced to 20 minutes. The correlation between CO2 concentration and test results of pupils' performance test results is noticeable but not definitive. It indicates that at increased CO2 levels the performance lowers - when the concentration of CO2 corresponds only to the Category 3 norm, the lowest results are achieved while the best results are when the CO2 concentration level corresponds to Category 1. To improve the study, observations of CO2 concentrations must be extended throughout the school year, as well as measurements in other classrooms in the school should be made.