That depends.
If all you had was a 1911, might as well just surrender. 'Cause you ain't got a chance.
They are heavy, you could kill someone with one if you hit them on the head hard enough.
All kidding aside I think it's a great point that in many cases people where able to stop the shooter unarmed.
If you decide to obey the law and not carry a gun in school ground you would still have options.Including a few lethal options like knives and other non-firearm weapons.
Even without any weapon.What if a school shooting situation the killer gets to a class with a locked door, he knows there are kids inside, he starts to kick on the door and he heards "Dont come in I have a gun!" shouted by the teacher.
I think the killer would easier run away or shoot himself.
Just like when they hear they that police is in the building, they give up before they actually see any gun.