@Spud
Oh wait. I have another idea. I think the principal didn't realize that Greer is a guy.
And once everyone did, the guard is to be stationed?
That works fine by me :) Instead of Greer's parents phoning the academy and letting them know, he just turns up, assures his parents he'll be fine, he can take care of himself (clearly he can if he's devious enough to smuggle pigeons in through windows).
Principal sees the name Greer, can't figure out if its a boy or a girl, sees Greer signing up for class in a skater skirt and puts him in the girls dorm. Two days later someone reports it to the principal that theres a guy in the girls dorm and Principal assumes thats utterly absurd, how would that happen, of course theres no guys in the girls dorm ... Then someone name-drops and Greer's file is pulled up where it clearly states "Male". Principal has a head-scratch moment and perhaps talks to Greer and the girls in the dorm and they've already made a stab at friendship and seem content so things get left as they are but because Principal doesn't want any trouble from concerned parents, a guard is stationed.