I'm sharing the following as a public service announcement.
I just finished a training (8 modules, with quizzes on each one, more than an hour altogether) on what your child and your teacher should do if there is an active threat to their school. (There may be some exceptions for young and disabled children.)
Times have changed, and tragedies like Columbine and Sandy Hook have been thoroughly analyzed, allowing law enforcement to come up with some new strategies.
We are no longer being told to lock the classroom door and gather students together in a corner. This has not proven to be the best course of action.
Now we are told to use our instincts.
If we feel we can evacuate safely through a nearby exit, we should do so.
If that is not possible, we are to lock AND BARRICADE the classroom door, and then spread out around the room. Call 911.
If confronted by a violent, armed intruder from which there is no escape, and only as a last resort, we are being told to use whatever means necessary (throwing chairs, desks, books, staplers) in an attempt to disrupt the thought processes of the intruder, and possibly to physically overcome him.
Most of the time, the violence is over quickly...before the police even arrive.
There have been cases where teachers and students have caused disruption to the attacker's intent, and saved countless lives.
A locked door is often not enough of a deterrent. Many shooters have either kicked open the door or shot the lock off to gain entry.
The same principles would probably apply to non-school situations.