Department of Earth
An obsidian spire with windows dotting the side. The windows themselves vary in color and shape, but they are all made of crystal, though whether they are colored quartz or real gemstone would be difficult to tell without the skill of a true appraiser.
It seems like an inconvenient structure for housing classrooms. Indeed, the building itself is hardly ten meters wide at the base. But soon you realize that the spire itself is merely a museum meant to showcase aspects of Earth magic. The true structure is underground, and the stairwell opens up to a massive underground cavern that functions as a lobby, with minecart-style trams to provide easy transportation up its slopes and steam-powered machinery driving all the clockwork. Not unsurprisingly, a large percentage of students here are dwarven, some belonging to the stout and muscular variety, others thinner and almost elfin in figure.
The layout of the chamber is as follows: A spiral ramp descending from a giant brass-colored drillhead below, leading up to the bottom layer where the newest classes are held. As the ramp ascends, the width of the chamber increases, housing more and more classes, and all of the offices are located on the uppermost level, clustered around the bridge which takes you from the stairwell itself over to solid ground.
The roof of the cavern is dotted with sparking diamonds reflecting torchlight. Looking up from the bottom, it would seem like looking at the sky of another world, not a world under this one.