That line of reasoning may work then again it may not work. However, if the discussion is focused on
"What would you have done if your were there?"
"What could you do if you were there?"
"If you were in that situation would you have desired to be armed to protect your child?"
"Would you have desired to have lawfully armed staff members available to protect your kids."
"Knowing what we know today, do you continue to rely upon the police to protect your kids in school?"
This discussion must be centered solely on the fact that the police can not protect your kids while they are in school.