This is well beyond my knowledge and experience but....
..the common theme isn't.
Planning, and that includes disaster planning, is actually a form of modelling.
Which means that you don't just plan for a specific point or problem.
You systematically work through every link in the chain, including the known risks and remediations, then examine each one for process, points of failure and consequences .
What could happen- -what could stop it happening-what needs to happen to avoid this-what would the results of it happening be. And that includes the consequences of the precaution. Even little things like, say a fire in a waste bin. It's all very well making sure that the alarms etc. will be working, doing the fire marshal thing, evacuating, calling the fire brigade, using the right fire extinguisher if safe to do so, but also remember to get the extinguisher replaced afterwards if you do use them.
In my case would not just have been [vacate school] but also once we've done this where we store the kids, how we contact the parents, how we keep in contact with them once there is a solution etc. what the solution could be
And yes, more than once I saw the failure to do those extra steps in process e.g what do we do if a school is out of action for several weeks and yes I did have the experience of trying to get the local authority to put plans in place for these elements.. And I've seen the failures of process because these details were ignored.
For example, a school vacated for a gas leak, so the normal fire planning worked beautifully, leading to kids getting out of the building really quickly and sensibly, but then being kept in the playground (highly dangerous if the thing had gone boom!) while the school tried to find a safe place for them, because there was no access to the parents' phone numbers and no plan in place for emergency evacuation beyond the fire drill.
Or two other times when a school has had to be closed at short notice and there was just nowhere for the kids to go for the several weeks required because no one had planned beyond the immediate crisis. Yes, it's a once in a decade event. But stuff happens. And the model needs to fit.