I can see how this would be very upsetting for you - I hate the thought of this happening to my kids.

I think you are well within your rights to approach the teacher and ask for clarification of what happened, and of what their policy is for dealing with it. I don't think it's necessarily for a 5 year old to have to go home and tell their mother about an incident - I would have thought a quiet word from the teacher when you picked him up would have been reasonable.

But - kids that age are still working out how to play together and learning how to respond to conflict and where the boundaries are. If this was a first incident from the child and the situation was well understood by the teacher, then being reprimanded and name put on the board could be a good lesson. If the child has a habit of beating up on other kids or bullying then the teacher would need to take further action.