Why is school security taken so lightly?

Here is a partial list of things we protect with armed, trained law enforcement: Our politicians, our courts, our airports and our military bases.

Here is a partial list of things we protect with armed, trained security: Our money, nuclear power plants and other critical infrastructure.

So why do we leave our children virtually defenseless against mentally disturbed monsters?

And it is not just the mentally deranged monster that we need to consider, we also have foreign enemies that have no objection to targeting children: http://www.johnstonsarchive.net/terrorism/wrjp39ch.html