I have mixed feelings on the issue. On the one hand, if the teacher wants to keep a gun in the classroom I think they should be allowed to. Only if they have their concealed carry permit and have undergone extensive evaluation and training. If the teacher is busy shooting at a school shooter who's getting the kids to safety? Additionally, the instant the teacher fires at the shooter or the shooter sees the teachers' gun they become the shooter's next target. If the shooter doesn't take the teacher out, they can't continue to shoot other people.

BUT if the teacher is shooting too that means more bullets flying around and increases the chances that a child will get caught in the crossfire.

On the other hand, I don't know that "lockdown" types of procedures are the answer either: now you have defenseless people trapped with someone with a gun intent on doing harm. In self defense training they tell you if someone is shooting at you run! quickly, not in a straight line and as far away as possible. Of course, also taught are ways to deescalate situations so that violence does not occur. I'm guessing that school shootings are not one of those violent situations that are fairly simple to avoid (like someone saying "give me your purse or I'll shoot" you give them your purse - violence avoided.

All that being said, I'm planning to get a gun for my own use for my own self defense. However, even in your own home justifying a "self defense" shooting is tricky and you're likely to spend some time in jail and have some lawyer bills. I can't even begin to imagine the legal problems associated with a teacher taking out a gunman.