Abstract Students at the public primary schools in Medan city, Indonesia, complete 90% of their learning activities in the same classroom, which, in order to save energy, utilises natural ventilation to obtain thermal comfort and fresh air for indoor air quality, which is the physical school environment that impacts the students’ achievement. Air temperature is a crucial factor that influences thermal comfort, while the CO2 concentration determines the quality of fresh air. This paper aims to describe air temperature and the CO2 concentration in an occupied naturally cross-ventilated classroom in a hot and humid tropical climate, in Medan city, Indonesia. The air temperature and CO2 level were simultaneously monitored for four days during the students’ lesson hours in a school classroom that complies with the regulations of the Ministry of Education of Indonesia about the infrastructure standards for elementary, junior high, and senior high schools. The results of this study showed that air temperature in the classroom exceeded the comfortable temperature, while the median CO2 concentration was below 1000 ppm as recommended by ASHRAE and WHO.