CFK,
I too have been in the "ticked off the teacher" situation with my 8 year old. Like your son, over the course of school, DS had lost any eagerness or trust that school was a place to learn new and interesting things. Instead his perception was that it was basically a torture session of unreasonable, antagonistic teachers that were basically out to get him. The work was mind numbingly boring (hence alot of basic math and grammar errors) and had no relevance.
His situation actually deteriorated to the point that we had to change his school this year. In retrospect the biggest thing I would have done differently was let DS know that he had our support 100%. Instead, we tried to balance his needs with the 100% accuracy first attitude of his teacher. It was a huge error on our part. Our son has been in counceling now for 6 months both individual and family to work through alot of the issues caused by the horrendous school placement for grades 1 and 2. Telling him he had our support didn't do a whole lot. Given him honest but level appropriate feedback on how we felt about his teacher, did help him to realize that that she was unfair to him and that he could do much more than she saw/allowed. For eg, we told him that her actions toward him were inappropriate for a teacher and gave him specifics. However, that did not mean that he was allowed to act inappropriately towards her. It really came down to learning that the world isn't fair but that we were in his corner. He would have to do his part to complete the work and knock off his attitude, but in return, we would provide alot more afterschooling opportunities (like your tutoring) and lighten up on the minor stuff.
Nothing we could do, changed his teacher's attitude. Since his teacher was also the owner of the school that led to a complete change in his education. Knowing that we weren't going to be able to change it, we focused on how to make it possible for DS to get through it until the end of the term. I hope that you are able to make some progress with your son's teacher, but from the phrasing of your post, it sounds like he is one of the 100% accuracy must be a "perfect" student to be considered smart. It is also likely that he has never seen a student like your son before and has no training on how to teach him to his level and needs. It is therefore easier to say to your son that he isn't as smart as "they" say than to figure out what is really going on. Some teachers just can't accept that the highly gifted really do learn differently. Just curious if you have heard the ADD/ADHD suggestion yet?
One other thought, can you advocate with the principal to have some in service training at the school on how gifted children present in the classroom? Something geared to helping all the teachers maybe find those children that might otherwise fall through the cracks? I'm not in the education field, but after his new school found out about DS (and the nightmare that he was coming from), they did a half day in service seminar on at risk students and identifying traits. Part of it focused on gifted and in particular how many present as careless, unmotivated, and similar to ADHD.
I certainly wish you the best and hope that your son's situation improves! Seeing the light dim in my son's eyes at the thought of going to school was heartbreaking.