In Latvia, there are close to 700 schools where more than 200 thousand pupils study. The majority of the schools are still not renovated and operate without proper mechanical ventilation. The classrooms are often ventilated only during the breaks or, if the outside parameters allow, also during class times by simply opening the windows. This can lead to increased CO2 levels and reduced IAQ and thermal comfort. To determine the IAQ and thermal comfort of a typical classroom two CO2, temperature, and relative humidity loggers were located in a class. One of them was placed near the external window while the other was on the opposite side of the room. In addition, the external meteorological parameters were obtained from the official national database. Also, an anonymous survey on how the pupils felt regarding the overall IEQ in the classroom, their thermal sensation, lighting, noise, and smell pollution during the lessons was performed. The analysis of the results shows that the CO2 concentration constantly exceeds the suggested limit of 1000 ppm during class times, reaching a maximal value of 2542 ppm. Depending on the specific day the 1000 ppm limit was exceeded from 25% to 45% of the daytime. The indoor temperature was in the range of +20.1°C (68.2°F) to +28.8°C (83.8°F). With the daily averages around +24°C (75.2°F) to almost +27°C (80.6°F). At the same time, the survey results show that the pupils are generally in acceptance of thermal comfort – 75 % of the answers were in the range of -1 to +1 on the thermal comfort scale. Also, the IAQ of the room was evaluated as neutral or satisfactory by 76% of respondents. To improve the study, measurements in other classrooms must be performed and the survey period extended throughout the school year. Also, a similar study in a mechanically ventilated classroom should be performed and results compared.