Indoor air quality (IAQ) control in educative centres, where students spend most of their time, is essential. The presence of high levels of contaminants can impact the academic performance of the students and, ultimately, their health. A study has been carried out to assess the IAQ of higher education classrooms with natural ventilation in order to quantify the exposure of the occupants to certain contaminants during the cold season. CO2, PM10, PM2.5, PM1.0, and volatile organic compounds (VOC) have been measured. IAQ has been monitored throughout the normal functioning of two students’ groups for their whole academic period in a week, characterizing occupancy and distribution. Ventilation was made only occasionally by manually opening the windows. Also, during break periods when students move from one classroom to another and doors remain open, producing some air change between the classroom and circulation areas, and because of air infiltration. The airtightness of the classrooms was measured by means of pressurization tests. High concentrations of CO2 during the whole period were obtained, concluding that ventilation strategies need to be implemented to improve IAQ.