I think that the case that an armed SRO would ever prevent any deaths is a very tenuous one (akin to how many deaths are prevented by taking off shoes at an airport), but the case that eventually human error will cause injury in a school even with well-trained SRO’s is inevitable with enough time (taking off shoes at an airport doesn’t kill anyone).
When we’re talking about lives, I think it has to do with morality. How many lives is this school district willing to spend on unnecessary security theater? If it’s any, that seems deeply immoral to me. The people who freaked out over Ebola didn’t risk lives by winding themselves up.
It’s a trade-off. The benefits of kids playing generally outweighs the risks. The benefits of giving assault rifles to school guards are basically nonexistent, and the risks are very real (hundreds of accidental gun deaths every year, and unless you want to “no true scotsman” it, some of these are from presumably responsible gun owners).
