"Bad Notes"

So in the past 5 days of school, Griffin has managed to get into trouble 3 times. I don't understand why he is acting out at school. The first day he pinched a little girl for not sharing her toy, so we had a long talk about not pinching and how it hurts and he wouldn't want someone pinching him. He seemed to understand what we were saying, so we thought we had made some progress. The next incident was with peeing on the playground. And now yesterday, yet another note and a trip to the principal's office. Apparenly there was a little boy on a bike and Griffin wanted it. Since the boy wasn't willing to give the bike, Griffin took a rock and hit him in the head. He lost the rest of his recess and he had a note sent home. The principal actually brought him to the car yesterday to explain what he had done. I never thought he would act like this at school. He has attended a two day mothers day out before and this never happened. I guess he is definitely going to make Gabe "pay for his raising" so to speak. Today when I dropped him off he went in and apologized to his teacher and the child, so I am praying there will be no more "bad notes" from school~