Couple of quick thoughts until we learn more.......Make sure he has a good nights sleep.
There are sets of books aimed at this age group that are meant to be read aloud to them again and again and then talked about and practiced at home. A good start would be 'hands are not for hitting" found here - http://www.amazon.com/Hands-Hitting-Ages-Best-Behavior/dp/1575420775
And you will find many more if you scroll down.
Also, and this is really important. Do you have the same problems with him at home. If you do, then something like ADHD is always possible and you can contact me here to discuss this -
http://www.medhelp.org/forums/ADD---ADHD/show/175
If it primarily happens at school, then something else is going on.
I would suggest visiting the school to observe him.
Oh, and by the way, at his age bribes will not work! Kids of that age just don't have the ability to retain information long enough for a bribe about an action that may happen hours later to work. Short term - say, in a supermarket, yea - anything much beyond that no way. Also, punishment at home for something that happened at school won't work. You need to help him to change his behavior - which is one reason I recommended these books.