Size of the School

Ok, so I wrote the books before I figured out how large the school had to be to support it, but luckily it works out pretty well based on the amount of running around Fiz does. The following is an estimation based on how large the farms would need to be to supply the food needed for the student body and supplement the Order Army.

The school is at least 3 miles in diameter, with the farms covering around 1.7 square miles. To run around the entire top of the caldera wall would result in a 9.42 mile run, and the total area that includes the school, the center, the forest, the river, the farms, the other spoiler things, is around 7ish square miles.

That gives the Dungeon, 7ish square miles per level (because it goes down into the ground).

How does it not undermine the school and collapse the entire thing?

Its magic, the answer is magic.