I can't side with this argument either.
A teacher's job is to teach, not to be a security guard, bodyguard, or soldier.
They're already overburdened as it is throughout the day because they must take on the role of a mother, father, role-model, coach, adviser, analyst, actor, coordinator, mediator, secretary, tech support, and writer to be effective at their jobs.
They don't need more responsibilities to do their job better.
Would you ask a doctor, minister, senator, city mayor, baby sitter, or tour guide to act as a bodyguard too? They are leaders of large groups of people as well.
Should they too should be armed in case someone compromises safety at their workplaces when they are busy doing their actual jobs?