Don't you think it's a sad state of affairs when
1. such security guards are even required in the first place
2. you ask this question as if it is the default
Even unarmed security guards at schools is COMPLETELY unheared of in Belgium.
Except for a few jewish schools, which have been guarded by soldiers for a few months lately due to terrorist threat levels which included specific intel of a desire for terror attacks in jewish schools, synagoges, etc.
But that's not really the same thing imo.
The thing is though.... you are talking about armed security guards to protect your children from.... your very own people. Or even just other children. :-S
I get it though.... If I would live in the US in this day and age, I'ld feel more at ease if my child's school was guarded like a fortress. But that doesn't change the fact that this is quite a sad state of affairs.