My DC is five and in year one. They have a group of friends and I've never had any problems with their behaviour.
However I notice on the lovely Facebook that there have been two parties to which DC has not been invited. I'm not that bothered - but bothered enough to wonder whether I should be. The children whose parties it was are not close friends with my DC but all the other children are good friends with my DC.
Disclaimer: I had a shocking experience with DC1 and one school mum who ganged up on me about my job so I'm paranoid about making sure my children have as wide a social circle as possible.