A few comments:
1) Remember, the antis are never talking about a terrorist take-over scenario. They just don't want potentially dangerous teachers with firearms. And anyone with a gun, automatically, because they have a gun, is dangerous in their minds. This is consistent with their overall we-are-the-enemy, blame America and Americans first attitudes. The antis always say something like, "Well, what if a teacher goes nuts..and shoots up the kids. Kids and guns don't mix."
This is classic 'projection' and says more about the unstable nature of the person who would say this than kids and guns in school. The simple reply would be something like: "If you think that your kid's teachers are actually capable of killing your kids, then why would you EVER put your kids in their care."
Now back to the terrorist scenario. The next point has already been made and it's a good one:
2) the endgame for terrorist is to KILL EVERYONE.
So the risk of a teacher being a danger (missing the bad guys and hitting a kid or something) is negligible. Terrorists killing everyone being about as bad as it can get, there is no risk or danger of an armed teacher making it worse. Terrorist: "Darn, I was going to be nice and let everyone go and surrender. But now, because a teacher tried to resist us with a gun...now I'm mad. And now I'm going to have to kill everyone." Of course this idea is ridiculous.
The logic now being firmly established, teachers with guns in a terrorist takeover scenario can never make the situation worse only potentially better. Remember, terrorist are bullies. Bullies prosper in exact measure to a lack of resistance to those they are bullying. In a terrorist scenario, some resistance is always better than no resistance because no resistance leads to everyone being dead.
General comment. The entire 'kids and guns'...'guns and schools' argument is based on "false parallelism"(watch for this in most leftist rhetoric). In creates false disaster visions in a listeners' minds. Always, the way to combat false parallelism is to first, point out the falseness of the parallelism, then correct the parallelism with a proper comparison.
For example, a good response to "Kids and guns don't mix", would be something like, "I care about my kids enough not to leave them unprotected". This points out the reality-based parallel that the antis are creating....
kids+no guns=unprotected, vulnerable children.
Then, now that the false parallel has been point out, you follow it up with a proper parallel. Something like, "Sorry, but I just think that dead bad guys equals the restoration of happy, safe classrooms. And it kinda scares me that you want to leave terroristic killers in control of our kids."
armed terrorists+no resistance=you agreeing to leave murders in control of our little children.
Now of course this won't convert anyone who is so far gone that they are pushing stupid victim disarmorment stuff, but it will reestablish operation control of the debate and help others who might be listening--many of which might be undecided on the issue--to see things in their proper light.
Guns in schools=safer kids.
Enough said.