Personally, I think that the teacher's response is way over the top. Honestly, I'm a little suprised about the grading. At our school, homework isn't graded, because some kids can have parents correct and explain, and others can't. I can understand a completion/effort grade, but not much more than that. I guess I just don't get the hoopla if a kid occasionally forgets. They like to say that they're preparing kids for the real world, but in the real world, people are behind schedule all the time. I understand that you can get fired in the real world, but no one gets fired for the occasional mistake and I wish school were the same. Sure, if it's the third time this week, make them stay in from recess and do homework, or lower their grade, or call the parents... but if it's the first time and his effort and grades are good, cut the kid some slack!!
Now, the only thing I did think of was if they were reviewing homework and for some reason this was worth a high grade, and she didn't want your son to hear the answers... well, then maybe. But again, it's HOMEwork - you or dh could help him, or he could look it up on the computer.
I just think that they're asking for a level of maturity that is hard to come by in a 9 year old. 9-year-olds occasionally forget, or write something down wrong. No matter how intellectually smart this group is, they need to be treated as kids (and not embarrassed either!!)
Theresa