The report presents results of a study of thermal comfort as a criterion for ensuring a favorable learning environment in school buildings. By combining objective measurements of the physical factors of the environment, PMV/PPD indices and subjective surveys, the influence of the microclimate on students in high school are analyzed The study includes objective measurement and assessment of the microclimate by the classical method and indices for thermal comfort and assessment of the subjective thermal sensation of students during the measurement. It is performed during the “cold” period of the year, which is characterized by an average twenty-four-hour temperature below 10ºC and covers 300 male students (14–16 years old) in schools in Sofia. Specialized equipment is used to measure microclimate parameters: temperature, relative humidity, air velocity, radiation temperature and PMV/PPD indices. Tools for visualizing comfort data of the PMV and PPD indices were used as well. To assess the subjective thermal sensation of the environment an anonymous survey among students of the mentioned age group was conducted with a non-representative sample. Conclusions have been drawn concerning the connection between the data from the two types of research, which provides information about the relationship between the values of the studied parameters of the microclimate in the classrooms and the ability of students to concentrate at the temperatures causing discomfort. Over 70% of respondents indicate that thermal conditions affect in moderate to very strong degree affects their attention and concentration. The existing limit values for microclimate could be used as a starting point for development a new legislation for schools environment, but other approach is needed for assessment and control thermal environment.