Story highlights More than 300 mass shootings in 2017, non-profit says

"Train yourself to work past freezing," expert says

(CNN) School fire drills became popular decades ago after several deadly fires triggered changes in safety codes. Today, teachers and children are preparing for something entirely different: mass shootings.

A gunman tried to break into a remote Northern California elementary school on Tuesday but officials say, the quick action of school officials "saved countless lives and children."

The building went on lockdown, a teacher rushed to block a classroom's door with a computer and students ducked under their desks. Those responses have become the new normal as more schools are being forced to adapt to more elaborate safety measures.

Two thirds of schools in the US conduct active-shooter exercises and nearly all of them have a plan if a shooter comes into the school, the Government Accountability Office found in a recent survey of schools.

"I think everybody, no matter where you are, needs to think about this. If you're in a school, in a college, if you go to the movies we should all be thinking about what are we going to do if a crisis breaks out right here," said Christopher Combs, FBI special agent in charge, after last week's church massacre in Texas.

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