The only other schools I can think of is the ones in Dragonspyre Academy, and then again they aren't really "hidden". Other then that, there aren't other schools. Only death and balance get the honor of that.
At least as far as I know. Perhaps someone else who is a little bit farther in the game then me could answer better.
there are only two other schools that isn't in ravenwood and that is death and balance but there are trainers that give's a card that some you don't get from your teacher you can buy them with training points but you can get them for free if the card that you want is your school