Indoor environmental quality plays a decisive role for the health of occupants and their overall comfort in a room. A well-balanced indoor air quality is important to reduce the spread of airborne infections, especially in frequently used rooms, such as classrooms. A comprehensive long-term monitoring was carried out as part of an observational study, considering three different types of ventilation concepts in classrooms: natural, decentralized and central ventilation systems. The monitoring lasted from October 2023 to April 2024 during the winter infection season. Various measurement data was continuously collected in eight classrooms across three elementary schools in Germany. This contribution analyzes the CO2 concentrations and air temperatures inside those classrooms concerning their specific ventilation system. Furthermore, appropriate contact switches at each window detect the window opening state to evaluate the influence of an additional natural ventilation. The arithmetic mean values during the school hours are considered for comparison. The lowest mean CO2 concentrations occur in classrooms with the decentralized ventilation units. On the other hand, the highest CO2 concentrations are measured for the central ventilation system. The classrooms with natural ventilation show most of the time acceptable values below a threshold of 1000 ppm. However, if the temperature is considered, the lowest room temperatures occur for natural ventilation. This study comes to mixed results for the mechanically ventilated classrooms. It shows that ventilation systems need to be controlled and operated correctly. The analysis also indicates that there are major differences between the classrooms within the individual ventilation concepts.