This is tricky territory. One one hand, you would like your child to treat the teachers with respect. On the other hand, most adults feel that questioning them is disrespectful. I think in elementary school it's best not to get involved in an 'but I'm right' battle with a teacher. After all, a few points off here and there aren't going to matter at this stage.
What about the larger issues - fairness, truth, Adults not getting to have their way 'just because they are adults?'
I think it's ok to say that inside each classroom, the teacher has to make 1000 decisions, and that their decisions are final if you are under age 12. You can say that most 7 year olds don't realize that there can be more than one answer, so teachers of 7 year olds don't usually have a procedure in place for dealing comfortably with students who correct them.
Just like I told my 7 year old that they are 'too young' for kissing, but that I would certianly tell him all that he needed to know when he was older, I think it's ok to tell a child that they are too young to challenge a teacher, and that in due time you will provide all the needed training.
I really think that a child gets more benifit from learning frustration tolerance than they do from assertiveness training in this situation during the elementary school years. If your child is particularly meek and respectful, then the situation might be different, by mine is bossy and critical by nature, and doesn't need any encouragement.
I do think that when the time comes, it's good to teach children the rules of the situation -
1) request a private meeting, do not start in unless no one can over hear.
2) start with a feeling statement. "I feel sad when ________ because _________"
as in, "I feel sad when I see my answer marked wrong, because I looked up my answer in the dictionary and I wrote down what it said." Shows paper. or 'I feel sad when I see my answer marked wrong because I looked up the Mars is closer is size, chemical composition, and year-length.
3) Get ready to listen, and listen.
4) One is only allowed to bring up any particular question once. If the teacher doesn't agree, the matter must be dropped.
As you can see, a 7 year old might want to start training now to have all these pieces in place by the time they are 12. My experience is that even if a 7 year old had the EQ to do this perfectly, it just would not be well recieved by many Adults.
Best Wishes,
Grinity