I know WA isn't the only place with a huge budget problem, most places are still lingering in the recession.
In the long run it's potentially cheaper to train and arm most/all teachers and school admins rather than hire additional guard personnel. The exposure of students to responsible armed role models in the forms of their teachers will also be a long-term cultural plus for the reasons that Gogodawgs stated above.
Also, from a tactical standpoint, having a few armed guards in a school is not as good as having many armed teachers and admins who are everywhere. Even with secure access points, if someone manages to sneak in, it will take the guards time to get to the crises. Whereas having many teachers and admins spread out will yield fast response time.
The problem I see with this is a matter of training and mindset.
Training:
We would have to set up the training where the teachers lock the classrooms down and fortify against an attacker if something happened. Having only a few non-classroom bound administrators who will search for an active shooter in a team rather than have all the teachers hitting the hallways, nervously popping shots at anything that moves. I don't think you can train enough teachers well enough to have them keep their wits about them to go search for a shooter.
I'm all for giving any teacher the option to have a gun handy while teaching if they are willing to undergo the background checks and training, but training, training and more training will be key to get parents to sign off on such an idea. God help us the first time any teacher hits a student while defending against an active shooter.
Mindset:
Most of our public schools are not a battle zone, and nothing even closely resembling this will happen in the mass majority of them. That said, most places will never "actively" arm teachers and administrators en-mass. Its not feasible in the current culture of this country, and the NRA know this. At best, you would likely see this done in the form of lock boxes placed in the trained teachers classroom. I think most of our public school teachers would opt out of such a program anyway, making huge gaps in classroom coverage. The current firearms culture in the US would never stand for teachers "packing" daily in our public schools, and there is no way you could make such a thing compulsory. I wouldn't even support compulsory carry.
Hell, we don't even carry firearms as teachers in the military, nor are they available in any way in the case of an active shooter scenario. We have to rely on first responders the same as anyone else. The only exception to this is during firearms training where all instructors carry "locked and loaded", just in case.
Ultimately it comes down to mindset and culture. I personally understand that these tragedies will continue happen regardless of gun availability (as they have around the world), and I am willing to accept the concept that a shootout is better than a massacre, but right now, the majority of our fellow Americans are not anywhere near that understanding or acceptance.
