The classroom was in a cavern underground. The classroom, and the entire colony, were in a sealed habitat that kept the air breathable and pressurized to the amount required for human survival. Any sudden change in that pressure was serious and could indicate a breach (a hole) in the habitat. Pressure doors were scattered throughout the colony that could seal an effected area off from the rest if necessary.
Air: Generation Mars, Book One
Coming in October
(image: Luis Peres Children’s Books Illustrator)
Reason.com 
