The Huffington Post recently ran a story featuring Wayne LaPierre, the head of the NRA saying that,
"Putting police and armed security guards in our schools to protect our children ... it's the one thing that would keep people safe"
Is a school with a regular police presence or armed security less likely to be involved in a school shooting incident?
This is not the same as this question about your average bystander, but about qualified, trained guards or police.