Do we want to add "Marksmanship 101" to the prerequisites for obtaining a teaching degree? After all, I certainly wouldn't want a novice shooter to be trying to "save lives" in a crowded school. Imagine the lawsuits! We could bring the public education system to its financial knees, as if they weren't already there.
I see suggestions, but they almost all involve more expenses taken on by schools. Where I come from, every single district is screaming for funding, and are rebuffed at every election. How can we even think about adding more costs? And what possible incentive is there to obtaining a teaching degree? It looks like a dream job that often ends up becoming a nightmare. Lots of time and money required for continuing education, dealing with unpleasant parents of the students, lack of classroom funding, people complaining that they earn too much.... 
