Don't know about other school systems / districts, but ours decided years ago to have only one entrance to the property and into the school after the kids have been dropped off their bus or by their parent. All the schools are surrounded by chain link fences, yet most all of the class rooms are open to a covered porch / sidewalk, like a one story, two sided motel. We have no halls in many schools so a crazy shooter could easily gain property access either during the day or at night after hours through the chain link fence.
The most vulnerable time would be when students are arriving or departing the schools.
It would take quite a few soldiers / officers to provide the needed level of protection for our schools.
I personally believe that if a teacher or administrator qualifies for a CCP then the next step would be to take an accredited initial SWAT type tactical and shooting training then a shorter version each year as an update ... If the teacher or administrator desires to concealed carry in the office / classroom. The children need not know who is or is not carrying. JMHO.
My wife is a teacher, she has a CCP, she is not interested in concealed carry at school ..... some of her younger and older teacher friends are. Most are not due them being liberal anti gun zealots who would prefer to sit down in advance with a crazy shooter and sing Kumbaya.