I would have made this into one big, fat joke. I would have poured ridicule down on them. Dad should have told the school not to worry, he only allows his son to carry blanks for his finger and can only target practice with adult supervision. I'd comfort the school and advise them he'll have his son holster his weapon. Better yet, I'd ask how they want to handle the "no guns in school" with his son...Like maybe cut his hands off.
And If I had a carried...it would be visible on my belt. These asshats need to see what a real gun looks like. You couldn't blow thier brains out, so what's thier concern.