Armed teachers in classrooms?

Thought provoking short 5 min video. Personally, I support gun control and don't have or want a gun. I also think teachers have a challenging job and should not be required to "carry".

On the other hand, if a teacher wants to have a gun, get trained on how to use such to protect their students in the event of an armed intruder should we deny them that op? Will that do more harm than good? Will having certain teachers carrying deter a school shooter from taking action in the first place?