There's a lot of misinformation swirling about on this topic so here's what I know for sure:
1. There should be no federal law requiring teachers to be armed.
2. There should be no federal law banning teachers from being armed.
3. The decision about whether to permit armed school employees should be made at the school district level. That way liberal and conservative districts can cater to their own sensibilities. Some districts may choose to completely ban guns and others may choose to arm teachers and provide armed guards. Some may choose a fortress made of beautiful daisies and others may take a razor-wire like approach. The bottom line is that school boards would be free to do whatever is right for their area and parents would be free to vote the board members out if they make the wrong decision.
Now here's what I would do if I were the principal of my local elementary school and tasked with providing security:
Before the school year started I would call of meeting all staff and tell them that since we have two wings on the school we should have two armed employees, preferably one in each wing. I would solicit only volunteers, nobody would be compelled to take this responsibility on. I would look for people that tended to remain calm in stressful situations and had past military experience or experience with weapons.
Since there will be only two armed individuals they might not be teachers. They might be custodians or administrators, but it could be teachers.
If they were teachers I would NOT provide them with handguns, they would be given one of the so-called high capacity assault rifles. The rifle would stay in a locked gun safe which is stored in the area of the classroom where the students huddle during the lock-down drills. Only the teacher would have the key. Why not handguns? They are too bulky to carry around all the time and too difficult to shoot straight.
I would have a small budget just big enough for two individuals to take the weapons to the range a couple times during the school year to shoot some practice rounds.
I'm not a gun enthusiast, at all, but I have shot guns various times over the years. Whenever I've fired a handgun I've found it very hard to hit the target unless I was very close to it. Add the panic atmosphere into the equation and anything beyond 10 feet is likely to be missed.
I've fired a 12 gauge shotgun a few times and the kickback is enough to knock you off your feet or knock the gun out of your own hands unless you are prepared. The wide spray of the pellets could be advantageous but the kick is too much in my opinion.
When I was in the USAF I fired an M-16 at the shooting range a few times and I think it is much better suited for the task. There is virtually no kickback. You can shoot it repeatedly and it remains solidly in your hands, doesn't bruise your shoulder, doesn't roar like a shotgun, and is very light and accurate. Since the chances of missing the target are great I would have a reasonably large clip. You wouldn't want a teacher to have to figure out where to carry the extra clips in a situation like Sandy Hook.
Lastly, I would advise the armed employees that they are not expected to act like a highly trained security force. The chances of them ever needing to fire the weapon are a million-to-one. They are to use their best judgement given the situation at hand. If they can run away, do that. If they can talk their way out, do that. Firing back is the last resort in a life or death situation.