Image copyright PENNSYLVANIA HOUSE OF REPRESENTATIVES VIA YOUTUBE Image caption David Helsel said he believed the "last resort" defence would act as a deterrent to would-be attackers

A Pennsylvania superintendent has described his unique measure to protect students from potential school shooters - arming them with buckets of stones.

Blue Mountain School District Superintendent David Helsel told state lawmakers earlier this month that some classrooms had been given river stones to throw at attackers.

He said the stone defence was intended as a last resort if evacuations failed.

The unlikely defence was revealed amid a national discussion on gun violence.

The national gun debate was re-ignited after 17 people died in a school shooting in Parkland, Florida, last month.

Image copyright Getty Images Image caption The shooting in Parkland led to renewed calls for gun control

Survivors from the shooting have organised a national March for Our Lives demonstration on Saturday, where they will protest in the US capital alongside other gun control advocates.

Several high-profile celebrities and politicians have met the students and are planning to support the headline Washington march.

Meanwhile, the Trump administration is planning to ban devices that can modify semi-automatic rifles to fire like machine-guns.

Attorney-General Jeff Sessions said changing the laws to ban "bump stocks" was a critical step in reducing firearms violence.

It will entail the surrender or destruction of about half a million of the stocks owned by the public.

Five-gallon buckets

"Every classroom has been equipped with a five-gallon bucket of river stone," Mr Helsel said at the state's House Education Committee on 15 March.

"If an armed intruder attempts to gain entrance into any of our classrooms, they will face a classroom full of students armed with rocks and they will be stoned.

"We have some people who have some pretty good arms. They can chuck some rocks pretty fast."

The unique defence initiative started circulating on Thursday after a video of the exchange was published online and picked up by local news.

The measure has been in place in the district's schools for two years already, Dr Helsel told Buzzfeed news.

He emphasised the measure was intended as a "last resort" and that lockdowns and evacuations were also in their planning.

He said he hoped to publicise the existence of the stones so they would serve as a deterrent for any would-be attackers.

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Parents and other community members gave the idea a mixed response, with some criticising the initiative, local media report.

Other US schools have implemented other security measures to protect against gun violence - including only allowing students to wear clear backpacks or banning personal bags completely.