B.,
I'm a teacher with over ten years experience in preschool and kindergarten classrooms as well as doing behavioral therapy for kids with AD/HD, autism, and other social issues. I've worked intensively with both parents and teachers to help kids with behavorial problems find their groove in a classroom setting, and in almost every instance, there are things that both teachers and parents could be doing to better faciliate the transition.
As a parent, one of the most helpful things you can do at home is to be consistent. Find out the language the teachers use to redirect behavior and start using the same redirections at home when applicable. Also ask the teacher if there are specific times of day or routines that seem to be the most problematic and then act out those situations at home. Autistic therapy utilizes something called social stories, but I think they can be helpful for all kids in new or difficult situations. It involves writing out a VERY SHORT paragraph about what to do in a new situation and gives a word for word script. For instance, the title of one social story could be "Sharing with a Friend." And it starts off like this:
"When a friend asks me for a toy I am using, I can say "Okay." Then I can give the toy to them. If I still want to play with it, I can say,"Can we play together?"
You get the picture. Rereading these stories (some moms have even illustrated them and bound them. They read them as bedtime stories or in the car on the way to school) helps the child have confidence and encourages developing more sophisticated vocabulary as well as social skills.
In extreme cases, I have asked parents to come and observe their child and actually step into the classroom to redirect behavior themselves. This is a double edged sword because it can make the teacher feel as if their authority is being undermined, so you'll have to make sure your teacher is comfortable with this. But the reaction a kid gets when Mom or Dad steps in and reinforces that expectations at school and home are the same is usually pretty valuable. I would use this technique sparingly, however.
Lastly, and perhaps extremely unconventional, but in order to facilitate a better relationship between teacher and child (and once again this depends on the school and the teacher), invite teacher to dinner at your home. The benefits of a united front are extraordinary and you'll send the message to the teacher as well that you are concerned and committed to resolving the problem. Somtimes that small gesture from you can add up to lots of extra commitment from the teacher in the classroom. You'll also get a chance to show the teacher how well behaved your little guy can be and the behavioral redirections that work well at home may give her ideas about how to deal with your child in class.
If you need more specific ideas, let me know. I'd be happy to help!