Sorry to hear this. I think it's very common for advanced kids to misbehave because they are so frustrated of being bored. My DS9 had a very good K (they were put in different reading and math groups right from the beginning and there was also a lot of free time), a somewhat OK 1st grade (teacher gave him enrichment stuff after he asked over and over), a terrible 2nd grade (no ability grouping and the teacher asked for everyone's full attention), a somewhat OK 3rd grade ( a bit of ability grouping and DS was allowed to just read on his own sometimes). So my first point is to try to find out how you can put your kid into a classroom or a school with teachers who are willing to be flexible.
My second point is that this is part of life that advanced kids have to know how to deal with. As my DS's music teacher puts it: he will not find many kids who are like him, he needs to know how to work with all kinds of people. We have been trying and things get better. DS knew from very early on that kids have different strengths and weaknesses and teachers need to spend a lot of time to help those who are struggling. He doesn't like this but he understands, especially after he made so many friends at school and he can feel for those kids who are struggling.
My third point is that we can teach our kids to get resourceful and make changes. For example my DS has talked to his teachers many times since 1st grade about what he wanted to learn and how he thought the teachers could help. It sometimes works.
For little kids in K it's tough. They don't know how to deal with teachers and school, and the shock of an unfit classroom really raises emotions. Just hang in there, many parents have gone through this, with time we all figure out some ways to improve.
