This is a good thread, lots of good points for why this isn't likely to be a good idea. I particularly like
@Moral USAF example.
I can see a potential deterrent, but think it's cancelled by the likelihood a student gets ahold of the gun, doesn't even have to take it from their parents house. Of course the damage would be limited in that scenario due to lack of ammo, but it's still a risk.
The question comes to mind. Iowa isn't the first to do this, how has it gone for others? They aren't talking about arming every teacher, merely telling those who want to have a gun that it's OK. I'm not spending the time googling it, but some sound statistics would be interesting.