First of all, I think it is best to have a policy of nothing inappropriate like a toy gun or weapon (and of course real ones!) should be brought to school, and during a PTO function.
I think it would be reasonable to approach the parent (since it is a PTO function) and the PTO has the resposibility (and liability) to operate safely.
The only exceptions to a "no weapons in school" policy might be a water shooting game during a field day or carnival, or possibly in a haunted house environment. And in this case, all weapons (or faux weapons) would be there under the auspicises of the PTO.
If a child arrives at a Halloween party with a weapon (which can be as simple as a stick that he waves), it's a bit of a subjective call. Can someone come in with a toy chainsaw arm, but not a separate toy chainsaw that can be swung about (we'll assume real chainsaws are definitely out
)? I would either
A. Require them to stow it in the car during the event
B. PeaceBond it, meaning secure it in a sheath or other similar fashion with a tie strap so that it can't be swung about
C. Make sure it is all soft foam and that the child knows that he must not swing it about (otherwise, they would be ejected)
But if the item is not essential to the costume (e.g. a Police Officer does not need a gun, a Fireman does not need an axe), then it definitely is banned.
Regards,
Ira