Leadcounsel,
Not really counting on a bookbag as a defense to pistol rounds without the bullet resistant panel. I am also a big fan of the box'o truth
. Did consider soft body armor when a specific problematic individual was involved but even there rifles and shotgun slugs can overcome soft body armor. Agree that the best situation is to flee but sometimes that is just not possible.
In extremis, it is a weapon just like a thrown chair, fire extinguisher, etc. If an armed intruder comes into the class then chaos is your best bet to allow some if not all to escape. A person blazing away with gunfire is not seeking hostages but to kill. IN such a situation you have to use what you have--I have advised students to throw computers, books, cell phones, etc. at a gunmen. What you have to do in such situations is go off the killers' script to create some opening for advantageous action.