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One method advocated by some schools included magnetically locked mobile phone pouches which cannot be accessed until the end of the school day.

Others gather around a rescuer who shows them a photo of the body on his mobile phone.

"I stopped him right away," she says - explaining how she then noticed a torchlight from a mobile phone above her head, and realised, to her horror, that he was filming her.

He previously pleaded guilty to failing to give police access to his mobile phone.

Describing the find as "crazy", he said he saw "details of the perimeter, the patrol, checking weapons in and out, requests for leave, mobile phone numbers, high ranking officers".