The difference between a Tool and a Weapon is how it is being used.
People have been Murdered by Hammers and Screwdrivers, even Rocks.
Does the School have an Industrial Arts Program.
All those Teachers are in Possession of Weapons.
For that matter does it do home economics? Confiscate that big chopping knife!
Don’t you also suppose they have some SHARP knives in the teacher’s lounge, for cutting bagels, etc.? Isn’t it reasonable that a student could force his way into the lounge behind a faculty member (assuming the room is kept locked and/or uses a security pad)?