On Armed Guards at Schools

I'm puzzled about why the National Rifle Association's President has been lampooned for suggesting that there should be armed guards in schools.  I'm not saying that I agree with the NRA's entire agenda, but what's wrong with schools having armed security-people?   

In the debate about whether or not teachers should be armed, the "pro" side appeals to examples in which a person with a gun prevented a mass shooting.  The "con" side then retorts that the person with the gun who saved the day was usually someone who was trained in the use of firearms----a police officer, for example.  Okay, so is the "con" side open to people who have been trained in the use of firearms being at schools to prevent mass shootings?  Isn't that what the President of the NRA was proposing?

An argument that I have heard against having armed guards at schools is that there have been schools that had such guards, yet shootings occurred at them.  Yeah, and there have been schools that did not have these guards, too, and shootings occurred at them.  Why should we focus on the former, while ignoring the latter?
Please feel free to comment, but I won't publish any comments that call me or anyone else stupid.  Plus, I probably won't debate, but I'll read the comments that people leave.