Responses
It's not so much about Kishima's size that acts as a bodyguard. It's about his history with Mifune. He's the only one who can control Mifune so that's why he's the bodyguard.
Gee, no mention about students assaulting teachers and everyone turning a blind eye to that though so I'm really doubting your skepticism of this 'silly fantasy world'. Ktbn.
I really disliked the teacher, but if they were going to make him so small and weak that he needs a body guard, why did they make him larger and in shape? It isn't because he is afraid of defending himself against students, because he is perfectly fine hitting his students or kissing them. And no one else in the school even blinks an eye at any of this? Just too far into the silly fantasy world for me.