Well, have 'gun free zone' signs ever stopped someone with intent to kill from killing? That is the only question that needs answering.
Can we imagine someone with intent to kill seeing the sign and thinking 'Damn, I'll just have to go someplace else."
The only thing that will assist in stopping a bad guy with a gun, is a good guy with a gun. Or knife, or sword, or sharp stick, or club, or whatever. Stopping them with pixie dust, or unicorn sprinkles, or good intentions, just doesn't seem to be the answer. If it ever has, I'd like to hear about it.