She most likely has a special trait that makes her the principal so when you deleted the other one she showed up. I suspect that you might have to keep deleting ‘other’ principals as the game doesn’t want played sims in those ‘special’ roles. None of the high school staff really have jobs there, they just have the special traits. I have a mod that prevents random sims from spawning so the high school staff will often show up as mixologists and vendors if new ones are needed. The game doesn’t see them as employed so it puts them in any needed roles. I hate it.