Here is an actual argument against "a good guy with a gun in our schools" instead of the obvious one.
Marksmanship is a perishable skill. you have to practice, a lot, to be good at it. You have to keep practicing to remain good at it. What is this mythical student Chad's ammo budget every month? And even if you are good at it you're only going to be shooting at someone after the shooting has started. So EVERYONE around you is going to be running for their life in random directions.
So in this vast sea of terrified children running in chaos I'm supposed to trust that Chad practices enough to only kill the shooter and not kill my kid or 5 other innocent bystanders that are trying to run away?? No thank you.
Even if Chad is simply hiding in a room with the rest of the kids in his class, there's still a super high chance of something going wrong.
In the last school shooting we all watched a video of students freaked out because the shooter was trying to get into their classroom. Same thing happens and instead of running away Chad opens fire through the door. 18 year olds don't really think things through first, so he's not really concerned who is actually on the other side of the door, he's just scared. In that situation I believe I read it was actually a cop who freaked those kids out, so now Chad has shot a cop.
Awesome solution Republicans.