Then don't post the signs and ask a question about them. Talk about the real issue that you think is important here.
Symbols mean things to people who give meaning to things
Weapons were already prohibited, but it is reasonable to ask whether the school should have had better security. Arming teachers would probably be a non-starter for most parents who sent their children there.
Maybe, maybe not. Maybe the parents would notice how much cleaner that school is from school shootdowns compared to other schools. Or maybe, they'll think "guns. bad" and, like you said, have doubts about sending their children there.
The mother of Adam Lanza owned a large cache of guns. Nobody knows why. Maybe they made her feel safer and more secure. Her own son shot her in the head multiple times with one of those guns. Those guns were not just useless to her for self-protection. They were the immediate cause of her death.
This is why we tell children no-touchy or else teach them gun lessons first. If your own son turns into a psychopath, you're screwed with or without a gun.
Also, you have to consider safety is not the only reason to own guns. You have to remember: hunting, entertainment, usefulness (i.e. making a sound to alert folks such as they often do for races, or to test stability of metal, etc.)
Now think about this rationally. Will a school be safer with every teacher carrying a gun? Those guns don't care who uses them. They can be used to stop a killer or supply a killer.
Think about
this rationally. Would a school at all be safe with a teacher who wants to kill? The teacher could bring a gun from home if they didn't arm the teachers automatically. The teacher maybe wouldn't even need a gun.
So, yes, if a teacher does go insane over something whilst all teacher being armed with guns, then there will be teachers with weaponry to take her down before she takes everyone down. If a teacher goes insane whilst no teachers being armed with guns, she can go into the art room and grab a blade or possibly go into the cafeteria and grab a knife, maybe she'll just decide to bring a gun to school the next morning. Who knows? But whatever she'll choose, the other teachers have nothing powerful to take her out with.