The current climate emergency context requires urgent action to reduce energy consumption in buildings while maintaining a healthy indoor environment. Promoting good indoor environments, in particular increasing ventilation, has been a prominent strategy to mitigate the risk of COVID-19 transmission indoors. However, this strategy could be detrimental to thermal comfort, particularly in the heating season in temperate climate zones. This paper presents the findings from a pilot project conducted in 2 primary schools in South Wales (UK), monitoring the indoor environmental conditions in a small number of classrooms that implemented operational guidance issued post-COVID-19. The study measured CO2, temperature and relative humidity. It explored the perceptions of the indoor environment by pupils and teachers, with a focus on thermal comfort and freshness of air. This paper presents the results of the monitoring study and discusses the behavioural adaptation strategies enacted in the primary schools to balance the requirement of reducing CO2 concentration levels while promoting thermal comfort.