I agree that there is a big difference between the two. I think the best approach when a child asks if Santa (or whoever) is real is to turn it back on them and ask them what do they think. If they really want to keep believing they will rationalize their friends' responses in some way. If they are ready to let go they will. They don't feel forced to stop believing because you have just ruined the magic by confirming the rumor, but they do not feel pressured to remain "innocent" to keep their parents happy either (or feel that their parents lie to them).
Yes to this

Presumably if they were discussing if characters were real it is because it is part of the third grade media sciences curriculum to learn too distinguish between fictional and non fictional chcarcters and stories. In that case, the teacher is doing her job. She is making sure ths kids understand. Perhaps the next standardized test will have a question about whether Cinderella is fiction or non fiction and if so these kids will get it right (well, all except for the ones whose parents undermined the lesson and told the their children the teacher was wrong and Cinderella is indeed real.)
OP--I don't think you are a bad person, or trying to undermine the teacher at all.
I think you were caught off gaurd and need to step back and think about this a little more clearly

Part of growing up IS learning how to distinguish between fantasty and reality. This is a very important skill to have. It is something most children should be able to do by third grade. It does not mean she will have lost all her childhood innocence overnight--or even that she will get any less pleasure out of WDW than she did before (I promise

).