I teach in a UK secondary school; the building with my classroom in was built in the 1960s, as many of them are. Ventilation is poor and after a 60-minute lesson with 25-30 students (age ~15) CO2 will be at 2000-2500 ppm. If there are back-to-back lessons it'll be 3000-3500. I can open some doors, but they lead to connected classrooms so this is only practical if those rooms are not in use. Retro-fitting an AC system is likely possible, but expensive.