Children should be taught to respect their teachers until or unless they give their parents reason to believe that they should not be respected.
Parents should encourage respect by being careful what they say about their child's teachers in front of the child and always finding out facts by a clear and reasonable discussion with school, before showing their child that they disagree with school staff.
The reason I say this? It's purely because of the attitudes of some children in school, from as young as six or seven years old. Some (very few) children are disrespectful and rude to teachers. This disrupts the lesson and interferes with the teaching of the whole class.
One or two un-cooperative, sulky or argumentative children can easily disrupt a whole lesson. It impacts the learning of every child. Much better to leave the politics out of the class, tell the child to behave and be respectful and, if there are still problems, deal with it by speaking to the teacher, head teacher or governors.
It's really simple things, such as telling your friend on the 'phone, "I don't care what the school say, I'm taking little Johnny for a term time holiday and they can stuff their stupid fine" or "That teacher is talking nonsense, I know my child best and they would never do/say that" and "I'm not letting someone else tell my child when to use the toilet, if they need to go, they need to go and that's that!"
Just some examples of issues discussed all the time on mn 
Fine to think these things, of course, and discuss them with family/friends and even to rant about schools/teachers. Just best not to do it in front of the children.
IMO