as a mother of three and nana of three I would first of all, insist that the school inform you straight away about any bad behaviour so that the reason for it can be looked into immediately, in case the reason is school related.
Now that you know there is something happening with your child that you want to sort out and you cannot put your finger on the reason you need to talk to the child's teacher, so make an appointment to see her/him. Discussions like this show your child and his/her teacher that you are taking it seriously and you want to find a solution.
There are protocols in place for just this sort of thing at the primary school where I work and generally they have made a difference to a child's behaviour, and rarely have I seen a child excluded for this type of conduct.
Does this kind of behaviour only happen at school? Does your child seem stressed? In my experience if a child acts out of character it is either because they are pushing the boundaries of acceptable behaviour to test the grown up involved or there could be a more serious reason, which you should, of course, think about.
oh the joys of parenthood!
good luck.