Does it really matter what it looks like, if you don't have any particular mental image for it already?
well, i'd like it to look advanced and modern, as it takes place in a high tech magical fantasy world.
Try to find pictures online - look up, say, "fantasy academy" or something - and examine them. You might find you like a certain aspect of what you see in the pictures and realise that is exactly what your academy should have. You already have some idea of what you want it to look like, (advanced and modern) and you'll probably find that you have more of a mental image than you realise as you go along. Sometimes I have to do this when I have a basic idea of how a room or building will be but am having trouble seeing the finer details in my mind. After looking at some pictures I might realise what I definitely don't want (and thus realise I was imagining [insert building name] to be like [insert description]/have [insert decoration/colour/etc]) or what I definitely do want.
Do you have any skill with drawing or photoshop? Whenever I have a scene that I want to make sure I keep consistent, I draw it. I find it easier to describe if I can see it, it also allows other people to see what you're envisioning do that they can better help.
