Imagine this scene: A neighbor is at your house, visiting over a cup of tea. You start feeling irritated and pressured when you realize you are running late for an appointment. What would you say to your neighbor? Imagine the same situation, except it's your child at the breakfast table. How would it change your response? Is it possible that you might respond in a more disrespectful way?
When a child is disrespectful to teachers or classmates, the first source to consider is the behavior of the adults in this child's life. Children who aren't treated with respect have no model for respectful behavior.