J.L.
If it's only once in a while, I would just talk to him, but discipline him in a gentle way. If he did it again, then I would get firmer taking away privledges--a bedtime snack, the t.v for the night or something that you know will make him think before repeating the bad behaviour. Punsihment doesn't have to be loud and mean. It can be gentle, but firm. I explain to my five year old and I don't yell, but I still discipline. Daddy is a pushover. Errrr...LOL. My little one will listen to me and respects me, though!
If the behavior continues, I would seek help. I would perhaps talk to other parents in the class or even stop in and observe, if possible, without the teacher knowing you are there. You want to make sure your child is not getting picked on by another child, the teacher or an aid.
It is normal for kids to misbehave or have a bad day here and there, though! I would address it, but not worry if it's only an occassional incident!