He is right
Why guard our money and not our kids?
And we allow those demented child- and granny-gropers in the TSA to carry firearms, but not a principal, not a teacher, either of whom can be trained and certified and can become as safe and reliable in the use of a firearm as any police officer.
He is wrong. An armed guard is an unnecessary and large expense and armed teachers works better. An armed guard is too easy to surprise with the first shot then there is that “gun-free zone” again. Multiple guards becomes prohibitively expensive. Just let it be known that teachers and staff may be armed. There will be no attempts at massacre again. Israel is the model. That does not preclude explosives but nothing really protects against that.