What grade is he in? Did he go to this school last year and were there any problems like this before?
If not, you need to look into what is going on in the classroom, find out if he has any new friends and that they are like, find out if someone is bullying him. You can set up a meeting with the principle and the teacher. Most schools are willing to do this to help the child. Let them know you are willing to do what needs to be done to help your child and ask them for suggestions on how to handle this. Usually, they have been to workshops and trainings on discipline and behavior problems.
If the problem doesn't seem to be with something at school, then you need to get to the bottom of it with him. If he won't budge you might have to get some tough love for him for a little while. Ground him if he gets in trouble at school, and reward him if he has a good day. Maybe start by the day...(if he has a good day with no trouble, take him for an ice cream after school) After a week or so of daily reinforcement, make it ..if he is good all week. Talk to the teacher if you try this, let her know what you are trying, and then ask for a daily report at the end of the day.
She will LOVE IT that you are taking an interest and trying to work things out!!

Trust me on that, I am a preschool teacher, and right now I am working with a special needs child, and we are having to do some behavior modifications. And his mom is all for helping, and we love it!!
If it keeps getting worse, you can ask for an SST, (student study plan) or a behavior plan, and ask for a behavior specialist. If your district has one.
And don't feel bad if you have to bring a counselor aboard. Anything you do now will help your child succeed!!
Lindy