Single pane (if thick) windows, very loosely sealing; in recent years possibly open for fresh air even in cold months. I grew up in a semi-temperate region tha

Single pane (if thick) windows, very loosely sealing; in recent years possibly open for fresh air even in cold months. I grew up in a semi-temperate region that only had at most maybe a month of really cold temperatures each year. Those under-seat heaters were still very nice to be near and also helped to melt the snow and slush into just ice on the floor. You should assume, from a heating and ventilation perspective, that the inside of a school bus is a mobile open air tent, but simultaneously also on the verge of being an enclosed space packed to the gills with people.