What you do not realize is "licensed" teachers have high training, qualifications, and the perfect mental health to carry a weapon.
There is also the option to not carry a weapon at all. That is a personal decision, as you said, it is not required, so teachers may refuse if they choose to.
Teachers will take their own time to be trained, they work with kids daily they know how to have self-control with their weapons.
Teachers will be able to defend themselves and their students just as easily as guards, but this brings up the topic of armed veterans guarding schools.