They are scared of all guns, and don't want any guards. I sat on a board; I was privy to the private debate on this. I was dumbfounded by how resistant they are to guards or trained armed teachers. They are more afraid of an every day guard than they are of a once in a million active shooter, so they are willing to roll the dice regarding their individual school. Instead, we bought tennis balls for the kids to throw at an active shooter -- I shit you not. (I said ball bearings might be better, but they thought that was weird.)
So, you have thousands of schools rolling the dice on this, counting on the likelihood that the next shooter will be at some other school, and not theirs. That way, they don't risk any liability for themselves. It is shameful, but it is really happening.
[Edit to add: I'm not even talking about whacko, child-grooming, raceism-training liberals. For every one of those, there are a dozen gun-fearing liberals who have never held a gun, and get nervous around them...they might just "go off" and hurt someone. The liberals on my board were great people who cared deeply for the children. They just couldn't face the issue directly when it came to actual life or death risk. Also, the local cop briefing the board was also a liberal, and only wanted guns in the hands of police, and smirked at the idea of guards, so he didn't help the discussion.]