Giving the good people of Gainesville the benefit of the doubt, I assume that the M4 Carbine is in addition to the handgun which the SRO has on his person at all times. This way, if there is a single shooter the SRO can use his handgun to diffuse the situation immediately. If the situation calls for more serious firepower, however, he has it at his disposal without needing to wait for backup.
In my opinion, that would actually be a pretty good emergency response plan, especially if willing teachers were trained in using the carbine so they can back up the SRO in the event of a serious emergency. Even better would be if this was in addition to the teachers being permitted to carry concealed weapons on campus. Ideal might be to have a couple of rifles/shotguns on campus in various locations (janitors’ closets, perhaps), which any trained faculty and staff may access.
Unfortunately it does not look like any of this is happening at this time. But it’s a good start.
I think that some of the decision comes from magical thinking. I have heard people say that someone (police officer) cannot defeat someone with an “assault weapon” because the AW is more powerful.
Some people imbue the rifle with some sort of mystical or magical power, that can only be defeated with an equal power (like Dungeons and Dragons, Magic The Gathering, and other roleplay or mystical games).
So they fear an AR, so they buy an AR to match its power.
Naturally within 25 yards or so, a person with a 9 can take down even someone holding the all-powerful Assault Weapon.