With that particular one, I would be most worried that the plans would be full of PC and/or just plain ineffective strategies. And since implemeting "guidelines" from the government is often tied to funding, everyone could be using substandard plans. Moreover, I would have OPSEC cocerns, as a prescribed standardized plan/training for all public schools and first reponders could potentially make it easier for the really bad guys to stage an attack.
From a Belslan style super massacre, yes, I agree.
However, from the disgruntled/crazy spree shooter POV, just the deterrent effect that all the kids will be locked up, and more "resource officers" etc. might actually work to a degree.
However, simply eliminating GFSZ's and allowing anyone with a valid CCW, (and making it impossible for the schools to prohibit it for employees or parents by policy) would probably work better, witness how many theaters the Aurora shooter passed up to find the posted one.