Hi there!
First of all, good for your son for trying to stimulate his mind and working ahead.
I missed how old he is (?), but I do have a few thoughts.
First, clearly there is no call for punishment of any sort. I'm also guessing from your post that the teacher isn't even recommending punishment. When you say that he's "in trouble" for working ahead, and that she is suggesting that only children who can work ahead without major errors do so, I'm not picturing her as being harsh here. I can picture her being perfectly reasonable. I doubt that she is putting him "in trouble" by offering detention or any other punishment, right?
There is a legitimate reason to worry about kids teaching themselves things, and a reason to worry that they will miss some key components if they self-teach. Sure, they can explore, but it is totally inappropriate for teachers to take that as a given that the child has truly and properly learned the topic (unless, of course, they really do score 100% on everything).
So, I think a few things you can consider: it is a great lesson to him to learn to read directions. If he wants to go ahead, I think you should appreciate the opportunity to teach him to focus on reading instructions. Assuming he truly "gets it" and is just mis-reading the directions, that is something BEYOND important for him to learn to do. As gifted parents, we are always seeking stretch goals, and opportunities for our kids to work hard at something. So this is a great chance to teach him the importance of following instructions. Of course, I think there is a chance that he's just stretching to do exercises that he's not ready for (as opposed to just mis-reading directions). That's not bad, just a different issue.
But this does offer an opportunity to discuss with the teacher that A) He fully understands the basic info that the rest of the class is learning, but B) He needs a TEACHER to help him with the further material--he can't properly learn it on his own. So hopefully she can step in to teach a more advanced group some more difficult material, but even if she can't, they perhaps she can provide some non-curricular but still stimulating math problems....challenging enrichment work on random topics like Sierpinski's Triangle, the golden ratio, etc that could be fun for him to work on, but wouldn't worry her that he is mis-learning some key curriculum information.
Anyway, just a few cents worth..... Good luck!! But rather than being annoyed at the teacher, I'd be grateful for the opportunity to work on a learning opportunity for your child--read and comprehend the instructions. This is a great way for him to demonstrate mastery and prove he's ready for more work. She's right not to trust him if he isn't getting above 95%--I think you might have a decent teacher on your hands!!!
