You bring up knives. Knives are symbols as much as weapons. I could stab you just as easily with a pencil as with a knife.
No system is perfect this system was brought about for a reason. I cannot stop a student from bringing a necessary tool to school. Yes you can kill and hurt with just your hands. This would still be punished in the old ways.
If you had zero tolerance for cell phones or for glass bottle drinks I would take action. But zero tolerance for things that don't belong in school is appropriate and has some studies that indicate it is working.
Yes there are some that indicate its a travesty but I notice they mostly rely on emotions not statistics.