I don't believe most mass shooters are looking for soft targets. Most target the place and people who they feel they have grievances. Some men go to their work place and kill their colleages. Some men feel they have a grievance with their wife or girlfriend. They target her home and family, or they target her work places and work friends, or they go to her church and kill her and other parishioners. They don't go to the local school just because it's a soft target. Most school shooters go to the school they attended because they feel they have a grievance there.

I also don't believe that arming teachers will make schools less of a soft target. Let's look at the most recent school shooting. Cage pulled a fire alarm to get crowds of students into the hall ways. Cage could have missed every target he aimed at and still have killed 17 people and injured dozens. If he missed his target the bullet would likely have hit the person standing to the left of him or some person 20 yards farther down the hallway. Cage didn't even have to aim, he could just spray bullets down the hall way.

Even if there was a highly trained armed teacher in the hall way he could have been 20 yards away standing in a crowd of students coming out of his classroom with crowds of students coming out of classrooms between him and the shooter. He'd have to spot the shooter through that crowd and hit that individual target without hitting any of the students in the hall way between. An extremely difficult if not impossible shot. Practicing at a gun range doesn't prepare you for this.

Most mass shooters want to die. They have already decided to commit suicide by cop. They just want to take as many people as possible with them. There's an incredible power that comes from deciding to die, there's no fear. The shooter can walk down that hall way spraying bullets without caring if anyone is shooting back. Where as those defending the school want to live. As bullets come towards them the instinct is to duck or hide. It's hard to calmly take aim and fire. The adrenalin kicks in with the fear. Their hand shakes. They really don't want to kill anyone. It's hard to kill someone who deserves it let alone one of the innocent students in the hallway. They are even more careful and slow to fire at the intended target. If some student in the crowed chaotic hallway moves in front of the bullet they are even more reluctant and more careful to take the next shot.

Think carefully about the details. Imagine it in your mind if you were an armed teacher coming out into that hallway and discovering it wasn't a fire drill but an active shooter situation.  I think anyone who plays out the details in a step by step fashion telling the full story would realize that arming teachers would rarely if ever be a useful response to school shootings.