Your math is just wrong as has been mentioned by several others.
OK, let's turn this around. What does YOUR math say?
Let's go WAAAAAY cheaper than I did last time. Say it's just $50,000 per school..... say one minimum-wage guard and some minor retrofits with security doors and the like. Well, there are still 130,930 schools. So, that's about $6.5 billion per year.
Now, how many lives will that save?
Again, we have a cap to work with, since we know we've been averaging about 36 school shooter deaths per year. We'll round up to 40. So, assume for the sake of argument the plan works perfectly, by saving every one of them without costing a single life (e.g., no incidents where our minimum-wage guard mistakes a kid with a squirt gun for an active shooter and kills him). So $6.5 billion divided by 40.
That's still $162.5 million per life saved.
Now, again, remember that when the government considers things like new environmental regulations, if the cost goes over a certain threshold, it's considered not worth it. In 2001, it was $6.1 million:
https://web.mst.edu/rrbryant/cba/Arsenic Controversy.pdf
That led even the liberal Michael Kinsley to grudgingly admit that George Bush was right not to tighten regulatory standards around arsenic in water:
https://www.washingtonpost.com/arch...ic-darn/566e8ac9-b39b-4d35-9eae-f8564bd724d3/
Adjusted for inflation that's just under $10 million today.
So, how about instead of pissing away that $6.5 billion to save 40 lives, we instead go with a bunch of programs that cost just over $10 million per life saved? In other words, we go with programs currently deemed just slightly too expensive to be worth it. Say $12 million per life saved. Well then, with $6.5 billion, we'd save about 542 lives, which randomly would mean around 120 kids lives saved, or three times as many as we'd save with school security.... with all those adult lives saved just thrown in for good measure.
If you don't like my math, what's yours? How much would you spend, and how many lives would you expect it to save?