The Dark Tower discussion

So I'm looking for a book I read while I had just started Middle School. It had a tower in it. That's what the setting was. I think I recall there being classes and ranks for your skill proficiency in various types of magic. It was a huge tower that held everyone. I seem to recall it was isolated or floating maybe? I believe it was. I never got around to reading the books after this one.

I also put it the topic down as The Dark Tower though I know that's not it, because a book was required in the "book this topic is about" field to post this.

Help would be very much appreciated. I've been trying to find this book for roughly a year now.