I don't know if there's a defensive solution to this. Gun-free zones are a joke, and teachers carrying could help or could not. I guess we do trust teachers with our children's safety and education, so trusting them with a firearm is not a huge leap, but I don't know if it'll get through, and what good it'll do is mostly speculative.
Call me naive, but I hate the "NEVER AGAIN" mentality. Your life, along with the lives of children, is in danger from psychopaths every day. If they want to kill you, even if you're a good fighter, *someone* can. Your pharmacist can poison you. A complete stranger could burn your house down while you sleep. The modern world is full of weapons, firearms or no. We will never clamp down on all of them, but this is not so terrible because it takes a uniquely horrible mind to do something like what happened in CT.
We live in a free society, and with freedom comes the fact that it could all come apart at the seams at any moment. However, it doesn't. The overwhelming majority of people don't want this, there are more good folks than bad folks. Think about it, what if 1% of gun owners went on mass shootings? That's 800,000 of 'em, by conservative estimates. Can you imagine? That's an apocalypse. But, it'll never happen.
Freedom means that we are all in danger to some extent, all of the time. The reason it works is because the overwhelming majority of us are not dangerous. We still have psychopaths, and they can still hurt us, but we form communities and help one another, and defend each other when we can (recognizing that we can't always). I think we should be really careful when it comes to building a society based on trying to prevent such occurrences by the truly lawless and amoral, and take comfort in the fact that something so horrible will, with overwhelming likelihood, not happen to us.
North Korea is perfectly safe (except from the gov't and starvation maybe), but it's still North Korea. People get hit by cars, but I still cross the street (though I always look both ways).