You might want to ask the teacher if she is of the opinion that your son is behaving differently from typical children in the class. Some teachers say this to every parent, but it does not mean that the children are doing anything unusual.
If she thinks its unusual, then you could begin a reward program for your son. Instead of punishing him for getting in trouble, you could offer him rewards for each day he does not get in trouble. This way, you are encouraging his positive behavior. This is what we do with my son-- it works somewhat. The book How to Behave So Your Children Will Too has some good tips.
Please try to remember that school is not the same as it was when we were young. These kids are in their seats almost all day. They are stir crazy. I am lucky enough to have a very understanding teacher for my kindergartener. She understands that these kids are going crazy being still all day. Remember your boy is very young and is really too young to be good all day long in a situation like that.