Pawpaw and Textension are correct. This is not a pistol vs. rifle toe-to-toe fight. Nobody is expecting Peterson, or anyone else, to charge the Bolivia army with Butch and Sundance. I have never been remotely in this type of situation but I was a teacher for many years and thought of this situation many times. The officer only had to find the shooter, engage, upset his OODA, and neutralize the threat or at least buy time until the Calvary comes or the kids can get away. Move to the threat but do so in a way that you can be effective. Blinding rushing in may do nothing more than get yourself killed and now you have squandered away the opportunity to deter or neutralize the bad guy.
We are arguing the pistol vs. rifle yet we expect our teachers to turn away an attacker with books and staples, or a fire extinguisher if they are so lucky to have one in their room. No, not all teachers should be armed. Not all are cut out for it. Let the ones that are willing and able have the tools to protect their kids. The teacher's first responsibility is to take care of the students in their immediate care, not search and destroy. If the opportunity arises to engage outside the classroom or hold down the hallway then that is a bonus. Staff that is more mobile, principals, custodian's, office staff, may have more opportunity to S&D but even then it has to be carefully calculated.
I've been thinking about your reference to Boyd's Loop. Very interesting.
I would say that the shooter shattered everyone's loop with his first shot.
So what does it take to get it back?
How do you regroup and regain the initiative?
Of course I disagree about it not being a rifle vs. pistol fight.