Although you feel like a drill sergeant, the teacher needs your help with this. Being the class clown is one thing, but openly defying the teacher is another. For basic safety reasons, children must listen to the teacher and do what he or she says.
Talk with the teacher and try to work out a positive discipline plan. Ask her to send you a daily note letting you know what your son's behavior was during that day. The note could be as simple as a "smiley" face or check mark. For young children the day can be broken into several segments (morning work, center time, special classes, lunch, etc.). When your son has three (or whatever works) smiley faces, he earns an extra bedtime story, a walk around the block with you, or having a friend over to play.
Ask the school counselor for help with this as well. He or she may be able to give your son some individual time or involve him in a small group on good behavior.