You identify your objectives (in your plan, not in your exposition).
You break the instruction into smaller parts, it might be spread over a number of discussions.
Verify and refresh occasionally that message is not forgotten, ask for exemples of succesful implementation if any - it could be the perceptions or intelectual discoveries - done by a child, even small do matter.
Let us imagine that schools started to teach that stealing is a good thing:
So you want your child:
1. to preserve a special rule that stealing is wrong,
2. you want you child to stay out of trouble and to learn what is useful and right in school, to respect teachers and not to quarrel
3. you want your child to benefit from difficulty - ie to learn something about the world and people
4. you want your child to respect you and be thankful for support
0. First of all collect the intelligence, go to school, PTA, talk with sympatetic parents, listen to your child, read the school materials if available. Do not be unduly suspicious verify that the wrong teaching takes place. Do not be confrontational, the best is other side does know even know that you have misgivings. 1. So you go first to explain child that stealing is wrong, you bring some story or example, if you child like to add to it or ask questions you encourage him/her.
2. then you talk about that people even with good intentions can be confused or ignorant, but that they still can teach us. You ask child what would you say if your math teacher said that stealing is a good thing? You would stress that he can still be a good math teacher. Ask also if math teacher can make a mistake in math? What will you do?
3. Tell to be diplomatic and respectful, tell that you do not need to confess everything to the strangers be they teachers or policemen. Show that discretion or even a lie might be justified in some situations. (you do not tell the thief where jewelry is)
Do not blame the teachers if they are mistaken, they are human too, they get orders and receive teaching guides which can be misleading.
4. Encourage discussion about human fallibility and imperfection of the world. (Only God is perfect).
5. Let child to express criticism toward you, be respectful if child does it, be ready to answer questions. Get trust and EARN it. Show that the truth and goodness takes precedence before your interest. Be fair, kind and loyal so children will be willing to ask you for help when needed.
Many conservative or Christian parents followed such or similar methods under Communism and managed to protect their children from propaganda. In the last resort like under Stalin or Hitler rule, when the terror is too intense and indiscretion could get you killed or children to be taken away, wait when children grow enough to be prudent and then slowly deprogram them. It was done in Soviet Union and Communism is gone.
Hey, after all public schools want to promote critical thinking! Don't they? So this is the opportunity.