If I was in charge of a room full of kindergarten kids I would not be happy if one of the kids was poking and hitting the other kids with a toy sword. It makes sense for the teacher to take the sword away before any fights get started.
I'm sure you wife put a lot of work into the costume and it looked great. But sending a kindergartner to school or a party with a toy sword is just asking for trouble and creating more work for the teacher. Think ahead and leave the sticks, clubs and swords at home next time.
That aside, would you post a picture of your son in his costume? Most of us would enjoy it.