In my opinion, the balance could have been achieved in a slightly different way, but that's a separate topic, not for this thread:)
I'll just say that air, earth, water, and fire have unique advantages that are specific to them, and if these advantages are logically arranged, they will balance all the schools:) in the sense that the player can choose their priorities, whether they want to be a master of air, earth, water, or fire. IMO, this would be more logical and also better for the gameplay. Everything is interconnected, and there should be a clear, albeit fantastical, logic in everything. There would also be rivalry between the schools, rather than their mixing, which, IMO, would be more interesting.