I think a gentle reminder to always respect others' beliefs (even if they seem ridiculous to her or are different than what she has been taught at home) is in order. Be it religious beliefs, Santa Claus, or whatever. If another kid asks her if she believes in fairies, or Santa Claus, she can say no. But there is really no reason for her to bring the topic up, or to elaborate on her own beliefs or get into an argument about it. The same social norms apply to adults. Unless I know someone well and they are a good friend, I'm not even going to bring up the topic of religion, for instance.
Agree except for the first sentence. "Respect others' beliefs" is one of those instructions which is often given but which, when you come to think of it, is either highly misleading or very dangerous depending on what you think it's supposed to mean. Therefore I wouldn't use that phrase approvingly to (anyone, but especially) a child. Instead, I'd encourage the same behaviour that blackcat lists under the heading of "be aware that these are topics where some adults like to tell lies to children for reasons that seem good to them, and people may get upset if you try to tell the children the truth". Much easier for your child to understand concretely, and more honest IMHO.
Moomin, I'm just as shocked as your daughter that the lunchroom assistant lied like that. I hope a teacher would have been more adroit at avoiding the lie direct! You might consider making that distinction to your DD, between teachers who are trained to teach children and can be trusted not to lie (though they might avoid answering a question!) and the other people who help look after the children.
