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Extensive field measurements have shown that school buildings ventilated naturally often suffer from insufficient air change rates (ACR) and elevated CO2 concentrations, which can result in decreased alertness, lower school performance and increased absenteeism of children. Moreover, the variable character of natural ventilation can lead to unwanted fluctuations in thermal environment, which is highly dependent on the actual window operation conducted by the classroom staff, and outdoor weather conditions. Mechanical supply and exhaust ventilation systems, if designed and operated correctly, can provide sufficient ACR, and positively contribute to desired thermal conditions. However, their installation is not always possible due to various reasons. In such case, it is necessary to explore alternatives to ensure sufficient ventilation rates and acceptable thermal environment. This paper presents an analysis of continuous CO2 concentration, temperature, and relative humidity (RH) measurements conducted in 45 Swedish primary school classrooms, equipped with different ventilation systems. The focus of the analysis is on the classrooms equipped with automatic window opening and mechanical exhaust ventilation. The measurements were performed during heating season and during one school week. Various statistical metrics were calculated, as well as the duration of periods when the investigated IEQ variables exceeded recommended value or were outside the recommended range. The variability of the thermal environment was also explored. All these results were compared with the remaining classrooms in the sample using different ventilation strategies. The classrooms with automatic window operation showed substantially lower CO2 concentrations than the classrooms with manual window opening, regardless of whether these had natural ventilation or mechanical exhaust ventilation. The CO2 concentrations were closer to the CAV/VAV classrooms. Temperature and RH were generally within acceptable ranges. Thus, the results suggest that automated window opening systems can improve the performance of natural and mechanical exhaust systems. The paper also addresses potential limitations and risks connected to such solutions.