Well, constrain to the underground aesthetic, intra-world portals are cheaper right? They could put the school somewhere else on Earth or another planet!
They could use future tech and hexes to create a flying Castle: http://tyleredlinart.deviantart.com/art ... -430375190 / http://fc01.deviantart.net/fs70/i/2014/ ... 748fme.jpg
Or inovate completely and create the first fleet school of magic http://tyleredlinart.deviantart.com/art ... -441604638
(i might be showing off cool pictures at this point)