Agreed. I say you call her on her bluff regarding the principal and entire school board (um, okay.....). Who knows why she's had it in for you for three years (is it jealousy by any chance?), but don't just give her the satisfaction of watching you quit. Or you can come up to her and be like, "so what is your problem with me, exactly, b**ch (ok, just kidding, minus the "b**ch" part). No seriously, maybe you could share with her the much more humane reaction you got from the coworker at the other school and ask her what exactly has been preventing her from being just civil with you, not to mention actually sympathetic.
Oh, and thanks for the support, rjessome.