Self-defense is not condemned in Scripture. And since the majority of teachers will probably not be Christians, it would make no difference to them one way or another. Is it sound policy? Yes, it is sound since it would be a huge deterrent to crazy killers walking into schools. And now that we know that the officers who were supposed to be protecting the students could not (or would not) fulfil their duties, all the more reason for RESPONSIBLE teachers to be armed. But there would also have to be some serious safeguards so that no crazed person can simply walk in and unleash mayhem, and no teachers would need to respond with lethal force (the last resort).