OP
My son is in exactly this situation and it is truly the worst thing I have ever had to deal with. My Ds is much worse than yours however as he is aggressive. He current has a statement of needs based on his behaviour at school. We are in the process of finalising a diagnosis of autism for him
Please don't worry that I am suggesting this is the problem with your son. I am not, but I am trying to say that there are many reasons this could be happening at school, many of which may not be in your control, despite what most other parents think.
Posie Parker has suggested some good questions to ask yourself, but if your answers to all of those is satisfactory (as our were) I would look further afield for reasons such as:
Does he have undiagnosed difficulties with school work? DS is very bright and has good language skills so on the surface he seems fine, in fact even very advance for his age, but he has difficulties with subtleties that other children his age might not have. He is also almost certainly dyslexic, which makes school very stressful and frustrating for him.
Does he have more difficulties with social skills than other children his age? Remember that at home these social skills are not tested to the same extent as they are at home.
These are questions that you may need professional help with. However before going down that route (before people start accusing me of pathologising bad behaviour), I would go into school and work out a good system of communication with the school.
Use a home school book and implement some home reward systems for your ds's behaviour at school as you have done for his behaviour at home. We have had some success with this with DS. To do this though you need to work closely with the school and you need a decent relationship with them. Give this a chance first before calling in anyone else.
If the school won't do this, I would seriously consider changing schools sooner rather than later. If you have a child that struggles behaviourally in school but not so much at home, it may not be anything you are doing but may be that he finds something in the school environment very difficult to cope with and the school needs to work with you to help him mangae this better. If they won't do this but continue to blame your son (and of course you by default) for all of his behaviour then you need to find a school that will take this on in a more positive frame of mind.
If after a few weeks/months of this you see no improvement then I would suggest asking the school to get the educational psychologist to see your son and to give some recommendations.