Normally I agree with Bostonian's POV, but I would be really torqued about my child getting that problem wrong in that scenario. That's nuts.

It would especially be worth my time because Iucounu is trying to get his son advanced.

That said, DD has been marked wrong when clearly right a few times, and I have never contested it. It's third grade, and she has straight As; stakes are low. However, I do tell DD that she was right.

I thought of another one, btw; DD was recently marked wrong on a grammar sheet ("Correct the errors in the sentence") for using the serial comma.