I can relate to this post, unfortunately......;(.
Gifted children ARE different, their personalities are different, their reactions are different.
My dd is 11 and in 5th grade and has had incidences of acting out in school when she gets totally frustrated. She knows how things should go and when they don't and the frustrations build up she can blow.
It doesn't help that some kids are so incredibly insensitive and mean....so we work on her behavior, role playing, etc. and it seems to be working.
Bottom line is, some teachers can't stand an overly bright or gifted child. It's an up hill battle, be prepared to advocate and stick up for your child, no matter what.
It can be done, I'm living proof. Is it possible for you to volunteer or drop in on the class to see what's going on?
Sometimes it's just not a good match between a teacher and a student, unfortunately. I always ask for "enrichment" work for dd so she won't get too bored.