I should have mentioned that in the first post. The principal gets his or her position much like the student council president only every adult in the world is eligible to try and fight their way to the claim. I bet he's S+ but never revealed his true power so just stayed in double S to win. Hmm...maybe he's the final boss? Or to achieve S+ takes a toll on his body so kills him in a heroic sacrifice and leaves us to fight the final boss.