[bravo whiskey]

I'll get flamed for this but WTH. First i believe trained teachers should be allowed and encouraged to conceal carry. Second i believe ccw parents should be allowed to carry on school property. That's not the flamed part.
There are two kinds of school attacks: one by a single deranged person and one by a trained strike team. Defending against the individual is a matter of making it hard for them to find soft targets. In this case, a good alarm system giving teachers and students time to secure and barricade doors so the shooter can't get in. If he does throwing lots of cans, chairs, books, etc. at him will most likely cause him to move on. Of course a ccw teacher would be best. The objective is to keep the shooter looking for soft targets until the LEOs arrive. The students are not going to hunt him down in the hallways with cans of food.
Against a trained strike team there are going to be a lot of dead people. Armed teachers would have to decide whether it's better to stay and defend the class or venture into the hallway. Most likely they will be outgunned and having an arms room with automatic weapons, as suggested by one Denver talk back radio host, isn't practical. Heavy locked doors would be a plus in this situation too. Throwing cans in this situation would not be practical either but it would beat huddling in the corner doing nothing.

[Gaffer]

I won’t flame you. I think non-liberal teachers should be armed, myself.

[If You Want It Fixed - Fix It]

Give the cans to the hungry. Bring a rock....better yet, teach Christianity!

No books to throw anymore, so they request cans.

[ProtectOurFreedom]

Huddled, doing nothing? Hardly. See what the World Religion teacher plans to do.

The W.H. Burns Middle School letter cited a couple of instances other schools’ employing canned goods in ALICE-inspired intruder drills, such as the Concord-Carlisle Regional High School in Massachusetts:

Each science classroom is equipped with cans of soup to be thrown at the intruder in the instance that the intruder enters the classroom. Many students and teachers have thought of more creative ways to handle the situation; textbooks, chairs, calculators, and other heavy classroom materials have been suggested as possible defense equipment. English teacher Kate Fleming even suggested equipping each student with a hardcover edition of Madame Bovary....World Religions teacher Ethan Hoblitzelle encouraged a calm state of mind during these drills by allowing his students to meditate.

http://www.snopes.com/politics/education/canattack.asp#i5mztGJDCWwPC3pb.99