Perhaps you can have a conversation with your child about the value of avoiding publicly-embarrassing people, especially those in authority (although I think it applies across-the-board). Just as you wouldn't start scolding him in front of all his friends, he may consider waiting until later to ask his teacher to clarify the difference between information he has received from her and information he has received from this other source. You can also talk about how people often are more able to understand and accept ideas if they work out the answer themselves, rather than having them told to them. Sometimes, when people are not given enough time to think quietly about an idea, their first response is to push it away, which often includes pushing the person suggesting the idea. It's almost a protective instinct, to avoid accidentally jumping into something harmful without knowing it.
This is a difficult position for a small child to be put into. My sense is that it is quite unlikely that the teacher will move from her position, as it is probably based more on fears and insecurities than on concerns open to rational argument. The second option, then, is to help your child begin to view the origins of this kind of behavior with compassion.
