I get the impression the main resistance to arming teachers is, teachers will be forced to carry and be duty-bound to confront the attacker. I don't think this is what is intended.
1. No teacher will/should be forced to carry (absolutely not). I would think this would be an option, and those teachers who volunteer will be the ones most motivated to make a difference. It is their choice.
2. I don't think it's meant for those teachers to then go after the shooter. That's a first responder's job. Armed teachers would be defensive: if the attacker tries to blast his way into a locked classroom, that's when the teacher can be ready to open fire, instead of s/he and students being sitting ducks.
Had, say, my school offered this option (ie, training + firearm), I would've taken up on it. The office in my main school is in the basement at the end of a corridor. There's only one other classroom down there, and all the windows are barred because it's in the basement. An attacker would just have to come down one staircase (right near the main entrance), turn left, and no one can leave those rooms or corridor. Sitting ducks.
Does it stink this state of affairs we find ourselves in, to consider arming teachers: hell yeah. But I would much rather be waiting for the attacker with a weapon in my hand than not.
Oh, and ban assault rifles. Banning isn't perfect, but it does help. It worked in the 1920s with tommy guns, and it works with modern weapons more powerful than assault rifles (eg, how come no one has attacked with RPGs?). But no one tactic will cover all instances. The other thing I would do is never show the attacker's face or name in the news (if they choose, show victims' families in private). Remove the "I'll be famous going out in a blaze of glory" incentive ... yeah, even if that means lower ratings for news show and unhappy sponsors.