You should tell them to listen to their teachers but if their teachers tell them to do something that is against what they learned at home that they should say no and if the teacher insists they should call you. It is the gift of no and every kid should have it in their pocket.
You should have, by this time, have already taught them that if someone tells them "not to tell your parents" that means you should tell your parents immediately.
That is enough for a five year old.
Yes, absolutely. Not that there’s anything wrong with the OP, but all that stuff comes in as the years (quickly) go by.