There is a great deal to be said, however, for learning the Japanese concept of "saving face", which is where that "perhaps I misunderstood" and "just to clarify" comes in. It never hurts to start a question with "I'm sorry,..." as in "I'm sorry, was it the size of the planet they were asking for, because I was thinking that the orbit and the atmosphere and the tilt of the axis (or whatever) were more similar in Venus...."
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I agree it is important for saving face and not alienating other people. Also, one thing we talk about a lot at our house is the idea of assuming positive intent in other people and thinking about the perspective of other people. The teacher is most likely not marking your work inaccurately as an act of spite. He or she likely is grading thousands of questions and mistakes will happen. If you start from the place where you assume they are stupid or out to get you your response is likely to be less than diplomatic. Kids should know even the smartest ones will sometimes be positive they got something right when in reality they misread the question or made a mistake. So, starting with the "I'm sorry..." or "Perhaps I misunderstood..." reflects an awareness that it is very likely that there may be a communication error that is causing confusion and it is best to make sure that's not the case before proceeding to the step or getting irritating or feeling like you are in conflict.
