I have a really different perspective on this one. Our son has heard a lot of praise about from professors and mentors for his willingness to disagree or offer another opinion. Too often students have had this beaten out of them early and unfortunately it stifles them and makes them passive learners later.
So, my approach would be to instead talk about the difference between big and little problems or errors. If the teacher makes a simple spelling error on the board that isn't worth interrupting her and it will likely irritate her. Everybody makes lots of little mistakes and people who wait for others to make mistakes so they can jump on it are irritating to be around. If on the other hand it is a more major issue and you want to say something learn to do it in a noncritical way. That may mean raising your hand rather than blurting out. Phrasing your opinion politely in a noncritical way like "I may have misunderstood..." "Just to clarify..."
