With time, it is well established that the citizens of Moperville are accustomed to some degree of strangeness. Granted this doesn't apply to everyone, but it apparently applied to enough teachers for Elliot to be able to pull this off.
Granted, I'm not sure what the Principal's plan was, here. "Hey, everybody! This is Elliot. He was transformed by Tedd into a woman, which is something Tedd can totally do."
Come to think of it, Tedd's transformation gun is supposed to be something of a secret. I doubt Elliot gave too many specifics, but still, dude, might not want to spread that information around.
Commentary added October 6th, 2014