I mean like, ya know, arm that gym teacher who acts like a rejected drill Sargent that is standard at every school.
I have other issues with the idea of cops in schools (and even more with untrained, armed civilians in schools), but that's actually secondary to the bigger issue.
How is this really going to stop someone from killing a ton of kids? A kid pulls out his gun in the middle of class, darts in front of the door, and just guns down everyone in the room. Even if he only kills like half of his 20-person class, that's still more-or-less the same number of fatalities as the average school shooting.
And sure, the rejected drill sergeant will come and cap the shooter, but that's minutes later after everyone's dead. So
unless there's literally no other solution upstream of the shooting which we can address, this sounds pretty stuffty and ineffectual as a solution.