We had this with DS's 2nd grade teacher last year. The teacher did it to DS in the classroom, and lashed out during parent-teacher conference when DS was not present. Mind you, DS is a very social and popular kid, has many, many friends, and the only trouble he had last year was that he was not the kind of kid the teacher expected from his cookie cutter. The teacher said to me several times through the school year that of course DS is smart "but many kids in my room are just as smart or even smarter, and none of THEM thinks my instruction is boring". The teacher had no clue that DS's math level was head and shoulders above what the other kids were capable of (this year's teacher totally understands this, though, and DS has been given some challenging work). DS is also a perfectionist and could have writer's block if he can't in his mind see his idea as being perfect. Once last year when DS had a writer's block and couldn't come up with something that he was happy with, the teacher said to DS that XXX (another kid in the same room) " is autistic and even HE came up with more sentences than you". DS cried on the spot. (And with the new teacher this year DS hasn't had writer's block again.)
We didn't go to the principal because we were new at the school and didn't know whether the teacher's attitude was typical among teachers in that school. While trying to find it out, we also looked around for other schools. But later on we figured out, to our relief, that last year's teacher is not typical. If we knew this at the time, we would have gone to the principal. It is true that gifted kids (or any kid who is atypical) have special issues and sometimes it is quite important for teachers to point out their shortcomings. But this doesn't mean abuse!!! I'm still sorry that I didn't stand up to last year's teacher the way I should. So I hope you do something. Any possibility that the principal is more understanding? Another school? I hope your DS will recover and understand that this is part of the price we pay for being different, and he has lots of people rooting for him.
