I think this is a special case, not the norm.
When the game generated a principal other than Mei for you, it didn't remove Mei's npc trait, it only added a second principal. (I have this case in the detective career, where I have two played chiefs. When I play Sim A, Sim B appears at the police station as chief and when I play Sim B, then it'll be Sim A. In any other case it is random, either A or B.)
Now the moment you deleted the second principal you were playing at school, and Mei wasn't loaded as part of the active houshold. In fact, the game thought Mei was in a different household from Molly's. Therefore she was eligible to appear as principal. Yes, I know that Mei really is part of Molly's household, but the game has trouble recognizing this. Example: When you are on a community lot with a sim, you may receive calls from their parents that live in the same household: "Why don't you visit me? I miss you". And then, when you return home, the parent will be overjoyed that you didn't forget them.
Long story short, I think this was an exception.