But the standard firearms training for law enforcement doesn't involve engaging active shooters, it trains on simple targets, this is why SWAT is an advanced course with far more expensive training methods. It is not cost effective to attempt to train every Rookie to be a school corridor sweeper while in the academy.
It isn't a volunteer armed citizen will purposefully aim at a child but will have a very cluttered engagement area with the strong possibility of children moving in front of and behind the bad guy. (this is why many SWAT teams use some form of carbine/MP5/, not because of any distance requirement but the much longer sight plane.)
The problem as I see it if these Sentinels are to be trained to the level of unpaid volunteer Reserve Officers they will have to pass a more intense backround check, beyond simple criminal past. The former Assist Chief of the Lawton PD told me a few years ago it wasn't the criminal back round that rejected most recruits to the force- it was the psych eval that cuts most recruits loose. Posturing politicians may be comfortable putting armed people in schools, but professionals should demand at least the same level of volunteer the academy does.
The problem will be the same that local police departments face, enough qualified people who will foot a rather steep bill to train to a level of competence beyond CCW or rookie cop. There is a reason most departments don't have their patrol officers rush into schools as they arrive, going in from different directions and in a piecemeal fashion is not a recipe for success.
So that leads us to the volunteers and their need to be able to do more than shoot. To not form a circular firing squad coz unless the badguy hits while all the armed teachers are in the break room they will be scattered around the school's grounds. The ability to move in a way so they don't just rush blindly to the sound of fire and add to the body count AND be able to hit a body armored shooter in the head after running and with all that fear/anger adrenaline pumping through their bodies- that takes a bit more training than what rookie cops get- hence that is why SWATT is an elite unit.
I'd say the difficulty many PDs have getting Sentinels for traffic/school zone/funeral escort/admin work the odds of there being ANY sort of viable concealed carry teacher REACT Force that arms, trains and then continues to practice is pretty poor.
Oh and how does a bad guy know who is armed in the school- any adult you see you put down, especially those not running away. You can shoot kids at your leisure afterwards.