You need to get started right now on a plan to change your child's behavior at school. Visit his classroom to see exactly how he is behaving. Of course, he may not misbehave while you are there. Also, ask the teacher to describe in detail the situations when he misbehaves. Once you have as many facts as you can gather, it is time for a conference with the teacher. It may be a good idea to have the principal or a counselor attend this meeting since more heads can be more effective at solving problems.
At the conference, everyone should work on a plan that describes what the school will do when your son doesn't behave and what you can do at home to modify his behavior. It could be appropriate to test for learning disabilities as continual aggressive behavior is one sign of a disability. At the end of the conference, be sure to agree on how your child's progress at school will be communicated to you.
At home, you need to talk with your child each day about what he is doing at school. You should discuss his behavior at school with him and try to find out why he is misbehaving. You must point out that his behavior is not acceptable and that there will be consequences at home for not behaving appropriately at school.
It may help to play school with your son so he can learn how to handle stressful situations. Also, if you believe that part of his problem lies in poor socialization skills, you should make sure that he has more time to play with children after school. It would be a good idea to consider if your child's "perfect" behavior at home is related to how he is treated at home. Does he have all your attention? Is he always getting his way?