If we shutdown all schools we would have 0 mass shootings at schools.

This is the only moral choice when dealing with absolutes.

Not really, that’s just one (extreme) proposal. I understand the self-gratifying impulse to write something like that though: it absolves the responsibility and work of addressing the problem, and frustrates the conversation away from finding solutions toward justifying the need to find a solution at all. It even makes you feel clever for a half-second.

A cute rhetorical parlor trick.

You said it was an absolute. Once we start weighing the costs against other things we care about like education it becomes a relative cost.

This is a principles based analysis. Pretty common in public policy.

Principle 1: Children should not be victims of arbitrary violence

Principle 2: Children should be educated

These are compatible principles. Your proposal (one of many potential proposals) is to undermine principle two rather than addressing the source of the issue which is violent third parties.

And to forestall any further swiss-cheese a priori logic: this is not something fantastical. There are other societies with have honored both principles one and two, and have experienced zero, as in ABSOLUTELY zero acts of mass violence in educational settings.

this is unironically my solution

policy decisions and budget allocation should an emotionless utility function that allocates what produces the best outcome with maximal efficiency.

To do otherwise is to be biased one way or another.