Gun control would have prevented the assistant principal from getting his gun in the truck, yes, but it likely also would have prevented the 16-year-old gunman from getting a rifle. It's kind of silly to argue that easy access to guns prevents crimes enabled by easy access to guns.
There are no gun control measures even remotely being considered which would have prevented the shooting in question (which was committed with a bolt action hunting rifle btw). And even more dangerous weapons are rarely impacted by such things. Norway has pretty strict gun control laws in place, more strict than even those proposed by the Dems right now, yet a gunman managed to kill 77 people in a mass shooting there.
The idea that gun control will prevent such things is a myth.
Now, if the assistant principal had stopped organized criminals (say, terrorists) from carrying out organized crime, that would have been a better argument.
If his gun had been more easily/quickly accessible, he might have saved more people than he did.