No guns allowed

The other day, when I walked Sofia to her classroom, I noticed the following sign on the glass doors. It is not like her school is in a bad neighborhood or anything, so I was kind of surprised to see it. I must have said something along the lines of "What's up with this sign?" because she told me in very matter of fact sort of way: "That means no guns are allowed".

"Who told you what that means?" I asked.

"I don't know. I just know", she replied.