Alternatively, the school is magical, so therefore doesn't have to obey the laws of physics to the letter. I imagine a hallway with several stairs. On one side, there are stairs leading to other floors in that same cluster of ten floors. On the other side, there are stairs leading to other clusters. Or in other words, you can get from any floor to any floor by only going up/down two stairs. Yay, (Plot) Magic! :D