You've received great replies already. I'll just add that you may wish to begin filling a notebook with dated documentation of each incident.
Work with your child to get a clear idea of background setting or circumstances, and also who said (or did) what.
Document in brief numbered bullet points, keeping each action in chronological order.
At some point, you may wish to provide a photocopy of any pertinent documentation to the principal, etc. But keep the original documentation of any incident(s) always in your possession - do not give or lend that, do not even lend that for school personnel to make a copy. Make copies yourself when you deem it appropriate.
