I'd try to be nice as described above. Also, I'm assuming this isn't the first test of the year? I think it's reasonable for the teacher to think that the kids know the drill for the test by now so I'd probably be a little upset with my kid for not playing the game by the rules. I have on who marches to her own drummer and she's had to learn some lessons about learning what the teacher wants. FWIW, her teachers responded better to her asking for the grade to be corrected vs me doing it and she's always been very good at it. Any chance your son would ask on his own at least as a first cut? (ie have him tell her that he ran out of space and ask why the zero). I do think if expectations were clear and minus a disability of some type my kid might be out of luck (ie I might not argue the point and chalk it up to lessons learned for her).
I have had to ask questions on a few occasions where more diplomacy was required and took the nice approach. Frame it as a question to clarify rather than a demand. Good luck!
