How's this for a deal. When a kid gets shot by a teacher in this school all the progun people will admit they were wrong and never bring up teachers with guns again. On the other hand, if a teacher with a gun stops a bad situation from happening all the antigun people will admit they were wrong and will fight for more teachers with guns at school.
I have a feeling one side would be willing to agree to that and one side wont because we all know which event is more likely to occur (neither event will occur though, the odds are greatly against it).
Neither side would ever admit to being wrong, even if both of the above situations happened, of course.
Allowing teachers to have guns in a classroom sets us on a very slippery slope that I'm pretty uncomfortable with.