I suppose a bunch of cans being thrown at an assailant might slow him down for a second or two while the teacher gets her gun out and shoots him. Given the right circumstances it might be enough of a distraction that maybe a few could escape who otherwise would not have. And fighting back by whatever means are available is better than just cowering and doing nothing. But to suggest something like this while not having the adults in the school armed with something that actually can stop the assailant is pretty stupid.
I agree that schools could be the target of a terrorist attack, or another mass murder by some crazy person who wants the liberal press to turn him into a celebrity. So not putting an armed responder in place is a betrayal of our children, and those who fight against such a responder being in place are in effect accessories to anything that happens because of the inability of the school to protect the children in their care.
How about this idea: There are a lot of retired middle aged men with carry permits. I'm guessing it wouldn't be hard to get quite a few who would be willing to be in the schools as a guard against this sort of thing. The sheriff or police could have some kind of training for them. Armed teachers and school administrators are a good idea too, but if they don't want to be armed, let those who choose to be armed be there to protect the kids.
It should be added that the Charlie Hebdo office had armed guards, so as we all know, guns in the hands of good guys are not a panacea. But it's still the best chance we have.