A Pennsylvania school district has placed buckets of rocks in all 200 of its classrooms to be used in the event of a mass shooting.
The Blue Mountain School District in Orwigsburg has also installed security cameras, secured building entrances and fortified classroom doors in an attempt to thwart future attacks.
Superintendent David Helsel said: "We didn't want our students to be helpless victims.
"River stones were my idea. I thought they would be more effective than throwing books or book bags or staplers."
February's massacre of 17 pupils and teachers at Marjory Stoneman Douglas High School in Parkland, Florida, sparked fresh debate in the US about how to prevent school shootings.
Hundreds of thousands of Americans joined rallies in Washington and around the worldon Saturday calling for tighter gun laws in March For Our Lives protests organised by survivors of the Valentine's Day attack.
Superintendent Helsel said the idea of equipping classrooms came from his reading of the active-shooter defence programme known as ALICE – Alert, Lockdown, Inform, Counter, Evacuate.
He said his school board approved the rock buckets before they were put in the classrooms at the district's five schools.
Parents in Orwigsburg, about 92 miles northwest of Philadelphia, have been mostly supportive, he said.
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Robert Conroy, director of organising with gun control group CeaseFirePA, said: "We should be talking about real reform of gun laws.
"It is so unbelievably tragic that our society has come to a point where schools have to arm themselves with buckets of rocks to defend them against active shooters."
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