A little history, abbreviated:
Mom, my aunt, and great aunt taught at the same little country school.  Mom told of the years (1929 ish) when was a teacher fresh out of high school and her sister taught up the road about two miles at another school.   Grandpa would get up early (5 am), hitch up the buckboard (sleigh in the winter), heat some bricks on the wood stove and take the girls to school to teach.  They would cuddle under the cow hide, feet on the bricks and drive the two or three miles to the schools.
Once there, she would open the school house to air it out, fetch the firewood and start the fire.  Fresh air heats faster than stale because of the oxygen in it.  Pump the bucket of drinking water and wait by the fire until the children arrived later.  At the end of the day, Grandpa would come to return them home in preparation for a new day.
The fresh air was also good because most mothers sewed the boys in their long handled underwear for the winter :)

(The above picture is of Aunt Florence Wilkinson Maseman, as teacher, approximately 1910)