Arming teachers, Jesus Christ what a stupid idea.
Lets look at the reality of it shall we?
First you've got to get them to agree, which I'm guessing they won't, you know with them being teachers, not Navy SEALS. Then you've got to train them in safe handling of firearms.
Then you've got to train them for the type of situation they might find themselves in which will be Close Quarter Battles, you know the kind of operation that due to the skill level and training requirements needed, only special forces units carry out.
It's not just your bog-standard Close Quarter Battles they will be fighting in though, they are probably going to be taken by surprise, where there are kids panicking, and screaming, and running in all directions, not to mention the fact they are getting shot at, then they will be expected to kill the attacker before he/she kills any of the students and they themselves will be expected not accidentally hit & wound/kill any of the panicking students.
I remember watching a documentary on the Iranian Embassy Siege in London, they were interviewing one of the SAS guys, who was taking about the training they underwent for this kind of situation. He was saying the average police marksman fired about 200 rounds a year down at the range, whereas the SAS guys were going through about 4,000 rounds every morning, or something insane like that. This was done to get them confident to carry out hostage rescue/CQB type operations, are teachers expected to go through this? Are they bum, they'll shown which is the nice end and which is the naughty end of a gun, then they will be expected to kill an attacker before any students are hurt, and if they don't and students are killed then we all know the blame will be placed entirely on the teachers shoulders for failing to protect the children in their class rooms.