## 1 Introduction  
Since the COVID-19 pandemic, the positive effect of higher indoor air quality (IAQ) for the mitigation of contagion risks among individuals has been emphasised. In this regard, schools, where children and adolescents spend many hours daily, have been under intense scrutiny. This younger population is more vulnerable than the adult population given their lower adaptive capacity and higher sensitivity to environmental changes [1]. It therefore becomes essential to understand the current state of learning environments, as an inadequate environment can negatively affect the health and academic performance of young people [2]. During the COVID-19 pandemic, natural ventilation of high-occupancy spaces became a particularly significant general action to mitigate the risk of contagion, as recommended by governments and the World Health Organization (WHO) [3]. In the specific case of schools, the WHO recommendation was to increase the total air supply to occupied spaces, prioritizing an increased supply of fresh outdoor air by manually opening windows and doors in classrooms [4]. The Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) also published guidance [5], advising 'to actively use openable windows (much more than normal, even when this causes some thermal discomfort)'. As specified in this guidance, a ventilation rate of 41/s per floor m\({}^{2}\) (corresponding to 8-10 l/s per person and 5 ACH)--in accordance with ISO 17772-1:2017 [6] and EN 16798-1:2019 [7]--may be appropriate to reduce the risk of contagion in classrooms. Although several studies questioned the sustainability of these measures [8], it was clearly necessary to improve the IAQ of educational spaces, which often fell short of the required health standards, even pre-pandemic [9]. In general, combined assessments of air quality and thermal comfort conclude that prior to the COVID-19 pandemic, comfort in classrooms was prioritised at the cost of high CO\({}_{2}\) concentrations. During the pandemic, efforts focused on increasing ventilation rates to reduce the risk of contagion, thereby significantly improving CO\({}_{2}\) levels to the generalised detriment of comfort conditions [10; 11]. In the research conducted in southern Spain by [PERSON] et al. [12], winter IEQ data measurements were evaluated from a pre-pandemic and pandemic phase. Before the pandemic, inadequate IAQ conditions were found, with a weekly average of 1000 ppm, while during the pandemic, a weekly average of under 700 ppm was measured but at the cost of a significant worsening of comfort. In another study in the northwest region of Spain, [PERSON] et al. [13] examined the effects of increased natural ventilation rates during winter in a school. In an initial pre-pandemic phase, excessively high CO\({}_{2}\) and total volatile organic compound (TVOC) concentrations were recorded around 90% of the time. New strategies were proposed based on cross-ventilation to increase ventilation rates and thus reduce the risk of infection, with no significant decrease in hygrothermal comfort. However, in severe climates, the implications of overventilation for the thermal environment of classrooms can be much harsher, as reported by [PERSON] et al. [14] in northern Spain during the pandemic. Similar results were found in pre-pandemic studies investigating schools in Italy. [PERSON] et al. [15] monitored a school in eastern Italy, where CO\({}_{2}\) concentrations of up to 2400 ppm were recorded, while comfort conditions were quite acceptable. [PERSON] et al. [16] evaluated different natural ventilation strategies in central Italy to determine their influence on indoor air quality. It was concluded that with short periods of window opening (the common strategy), the average CO\({}_{2}\) levels were around 1800 ppm, and to ensure levels under 1000 ppm, longer periods (at least 20 min) are necessary.