Minor note here about 3: From a legal stand point, accidental friendly fire from someone confronting an armed and active shooter is considered the fault of the active shooter (aka, the armed person committing the crime is the one held liable).
As you stated though, there’s not much a pistol can do to stand up against a high powered rifle. You also honestly don’t know who will freeze or act until their lives are actually on the line…
The other big problem with just arming schools though, is all it does is attempt to minimize the damage. The shooter has the element of surprise, so regardless of if teachers are armed or not, they can’t respond until after someone is shot (and in the case of a high powered rifle, likely dead).