2. Your policy is stupid because, as you have just argued, it makes sense to defend yourself and a can of soup is a poor substitute for a real defensive tool.
Could it be a reaction to a higher level policy?
IE The head of the school *district*, perhaps even state level declares 'NO WEAPONS!!!', complete with a set of definitions for what's a weapon(and canned food doesn't count).
Having to operate under these conditions, but unwilling to leave his school completely defenseless, the principle suggests this. Or he could be using it to get into the news so he can put political/social pressure on his district.
Personally I'd keep in mind that the foods edible and dual purpose:
1. Encourage kids to bring in cans of stuff that can be eaten directly from the can or prepared on site, such as with nothing more than a microwave and a few dishes you can empty the cans into. Voila, emergency defensive tool AND food in an emergency. Kids get locked into schools a lot more than they get shot up in them, after all. Some food in their bellies makes that more tolerable, even if it's just canned fruit and such.
2. Donate it to charity at the end of the school year.