Armed or not, the guard could have done the disarming.
This doesn't address the good with a gun vs a bad guy with a gun scenario, and thus is really just a moot point.
Don't get me wrong, I am all for armed guards at school (or SOMEONE being an armed defender), but this really doesn't fit the "here's my point proven".
If the guard had been forced to shoot or outgun the BG, then it would be a proven point.
But seriously. ANYONE in the vicinity would have been able to do what the resource officer happened to do at that point.
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