It's a hypothetical, which I grant you is a possiblility. But to try and plan out some mass killing, by guessing who's armed, ignores who else is armed, not to mention what ammo is the person going to use once they have supposedly wrenched their weapon away. Folks who carry concealed rarely have more than 1 magazine with them, then what? Most of the mass killings involved folks arming themselves to the teeth with multiple magazines and rounds. If they're being targeted to be shot 1st, again requires quite the guessing game, as to who else might be legally armed.
The risk-reward for someone wanting to shoot up a school is turned completely upside down, in the face any number of staff who may or may not be carrying a concealed weapon. Far too many examples of situations where livees were saved by the presence of a person with a CCW, not to mention the tragic stories of lives lost where someone who legally could carry, was not allowed to bring their weapon on the premises, because it was a declared gun-free zone.
Despite any number of hypotehticals that could supposedly thwart said Missouri legislation, I'm going to error on the side that really does have a far better chance at protecting more students from thje potential of a mass killer want-to-be. I agree it's not going to be a huge number saved, but that's partly because school shootings are not common at all. The kids win, responsible gun owners win, and common sense wins. The only ones that lose is the bad guy....and wound-be anti-gun zealots