The COVID pandemic has strongly affected daily life both in Spanish schools and worldwide. Providing the best environmental conditions for children allowing face-to-face learning with healthy and safe indoor spaces is a challenge. In the present study, empirical research about how these environmental conditions change with COVID is presented comparing the situation from March 2020 to January 2021. The methodology combines surveys conducted in nine schools with a case study in a selected school where a detailed monitoring of the building was developed during both heating seasons. This data analyzes the impact of the new COVID prevention protocols on indoor environmental conditions (especially those related to natural ventilation). Results show a mean CO2 reduction of 1,400 ppm, having in the second term values around 1,000 ppm, although temperatures diminished nearly 2 degrees C to mean values of 18 degrees C. Evolution of temperature and CO2 concentration throughout the day was also analyzed, being these indoor conditions especially important for the children with poorer health. Mechanical ventilation with heating recovery should complement natural ventilation, at least during the coldest months or hours of the day, although systems have to be carefully designed and installed to work effectively. (C) 2021 The Authors. Published by Elsevier B.V.