I'd give it days at the most before the weapons start falling into the hands of the students. Kids are smart and get everywhere. Kids can pick locks, crack codes, hack computers. They can do all that and be incredibly stupid at the same time.
This measure will put schools in far more danger than they are now.
Israel has a 60 day waiting period for gun purchases, you must be deemed mentally fit to own one, you are trained in its use and you get 50 bullets for the rest of your life. We should be doing all of this.
Remember when two trained police officers shot 9 bystanders while attempting to take down a man at the Empire State building last august? Doesn't really make the case for having armed teachers in a classroom, does it?[link]
The day of this (most recent) shooting, a man did the exact same thing in a school in China, except with a knife instead of a gun. 4 kids were wounded, 0 were killed. Guns are the obvious problem here.