A teenager stabbed his girlfriend 32 times during a frenzied cannabis-fuelled attack after friends wrongly told him she had Aids.

Edward Bell, 17, took the 15-year-old to a beauty spot where he persuaded her to take part in a mock bondage scene in which she would be blindfolded and hit with a stick while a friend filmed her on a mobile phone.

Instead, he launched the 15-minute knife attack which left the schoolgirl with multiple wounds to the face, neck, back and hands and with two punctured lungs.

Incredibly, she was still alive when paramedics found her hours later after Bell turned up at a police station to confess what had happened.

Prosecutor Ann Evans said: "Her abiding memory is of being repeatedly stabbed and seeing the light from a mobile phone in the background."

Bell, who admitted attempted murder, was sent to a secure hospital indefinitely today after a psychiatrist said he was a paranoid schizophrenic with a history of substance abuse.

Judge Michael Baker QC told him: "In an act which was plainly one of premeditated brutality you stabbed her many, many times and left her for dead. It was an appalling act."

St Albans Crown Court in Hertfordshire heard how Bell, a sixth-form college student from Hertford, had been going out with the boarding school pupil for a few weeks when an unfounded rumour began circulating among friends that she had Aids.

On December 19, 2006, he armed himself with a large kitchen knife before getting in his car and picking up his friend, James Galloway.

They then collected the girl, who cannot be named for legal reasons, and drove to the Waterford Heath nature reserve near Hertford where they went for a walk.

Miss Evans said: "Edward Bell suggested to her that they film her being hit with a stick.

"It was, she thought at the time, a joke and he assured her that he would not hit her too hard."

She agreed to allow Bell, who is now 19, and Mr Galloway to use her jacket to tie her arms and blindfold her.

Miss Evans added: "Once she was blindfolded she felt a blow as Bell began a frenzied attack upon her."

Bell later arrived at Hertford police station with blood on his hands and jeans. He claimed he had heard voices telling him to carry out the attack.

The girl, who described hearing the men laughing as they left afterwards, was rushed to hospital and has since had plastic surgery.

She still suffers nightmares, is unable to trust people, and refuses to wear summer clothes that would show her scars.

Judge Baker told Bell stringent conditions would be imposed if and when he was released from the secure hospital.

Mr Galloway, 18, who is from Ware in Hertfordshire, was accused of using the mobile phone to provide light during the attack.

He was tried last year for attempted murder but the case was halted after Bell gave an account in court that differed from his police statement.

A judge later ruled it would be unfair to hold another trial as Bell refused to co-operate with the prosecution. Speaking after the hearing, the girl's mother said she was "not the same girl since the attack".

She added: "My daughter never had a sexually transmitted disease. The whole thing was just a stupid kids' rumour.

"[Bell] seemed to have a good reputation. He was at college and as far as I knew came from a good family."

Police yesterday said tests showed Bell had smoked cannabis on the night he tried to kill the girl.

Detective Steve Healy, from the Bedfordshire and Hertfordshire major crime unit, said the girl's injuries were among the most horrific he had seen in more than three decades.

"It is a miracle she survived and she is still traumatised by the events of that night," he added.

"We hope she and her family can get some closure from today's sentencing."