You are being ridiculous and hyperbolic. You have lost all sense of what is an acceptable response to someone in a costume.
“But, but, but... someone in a costume could shoot up a school!”
So? Students with backpacks have shot up schools. Do we call the cops because you don’t know if a student with a backpack has a gun?
The proper response if you see someone who shouldn’t be there is to talk to the person and tell them to “move along”. That is what civilized society has agreed upon.
To be so gripped by irrational fear that law enforcement must be called as the FIRST action shows there is a lack of judgment and leadership.
In the case where someone already has a gun out as part of a costume, there would be less time to act, than if they had one inside a backpack.