Bullet-resistant glass and doors are a good idea, and lots of schools in bigger cities in Mississippi are already doing it. It is incredibly expensive though. At the very least, you could use security film by 3M
This film is also expensive, and certainly not fool proof, but minutes count in these attacks. Rifle and handgun shots would shoot holes in the film, so it would still take effort to break through
ETA... gun control, bullet resistant glass, and security all seem to be like seeing you have a leak in your ceiling bc the drywall is stained and just fixing the drywall. You still have a serious problem that will eventually show up again