Right, well.. why are you going in to school today? Do you have another reason for being there or is it solely to address this issue?
If you are there for another reason, I would be tempted to deal with that, then attach a note to the offending piece of returned homework saying something like:
"I am a bit unsure as to why this has been marked incorrect. My understanding of imperatives is [insert definition in your own words], which matches dd's work above.I would value some written guidance on the examples marked incorrect to avoid futher confusion".
If you deal with it face to face, you are going to get into a situation where the teacher hasn't a clue what you are talking about, will try to justify it using examples as per the other thread and you will have to explain where she is wrong in some detail which has the potential to be cringe-inducing in the extreme, particularly if she believes she is correct and attempts to justify her corrections. If you ask in writing, she will have the opportunity to go away and research imperatives and learn for future reference, and will be more inclined to accept your correction at face value, I think. She may even say, then, that it was just a miscorrection.. saving her and you embarrassment.