Continuous measurements of indoor parameters, including temperature (T) and relative humidity (RH) were conducted in a naturally ventilated classroom during the heating season. Thermal conditions were assessed using three newly developed 10-point thermal comfort indexes: TCI 1 based on hygrothermal parameters; TCI 2, calculated from acceptability of air quality (ACC) and the percentage of dissatisfied persons (PD); and TCI 3, based on Fanger's indices (PMV and PPD). The analysis showed that thermal conditions and comfort were generally favorable but some deterioration was observed during lessons and periods with higher CO2 concentrations due to student presence. During these periods, T and RH reached peaks of 25°C and 51.3%, respectively, with TCI 1, TCI 2, and TCI 3 reaching values of 5, 3.4, and 2.1. The results suggest that continuous monitoring of TCI 2 and TCI 3 which showed good correlation with hygrothermal parameters enables an objective real-time assessment of thermal comfort. This approach could be applied to optimize thermal conditions not only in the monitored classroom but also in similar indoor environments.