A teenager has been found guilty of murdering his "defenceless" classmate in woods.

Craig Roy was found guilty by a jury of killing 16-year-old Jack Frew by stabbing him 20 times and slitting the teenager’s throat.

The 19-year-old had admitted killing Mr Frew, but denied murder. The victim's family described the murder as "calculating and planned" while they also said Roy had "tarnished" Mr Frew's good name during his court evidence.

During the trial he claimed he had taken a knife out with him to meet Mr Frew in East Kilbride because he thought his fellow Duncanrig High School pupil was "blackmailing" him for sex.

Roy’s claim that he could not remember the attack was criticised by prosecuting depute advocate Jennifer Bain during the trial at the High Court in Glasgow, when she said his "amnesia is selective or convenient".

This was because Roy had previously told psychiatrists he was "furious" at the time of the murder, while Miss Bain said the teenage killer had subsequently lied "in order to minimise his role" in the death of the "defenceless" teenager.

Roy faces a life sentence for the killing on May 6, 2010, and he will be sentenced on March 1.

Mr Frew’s family issued a statement after the guilty verdict was returned.

It read: "This trial is now over. Jack's family have sat through eleven days of harrowing and distressing evidence. The verdict could have been none other than murder because that's what it was. Cold, calculating and planned murder.

"Not only has Jack had his life snatched from him but his good name has been tarnished by wicked words and lies which were unfounded. The family can only take comfort from the fact that those who knew Jack know and remember what he really was like.

"He was a happy loving vibrant boy who loved life and brought fun and laughter to every situation. Jack loved people, had the world at his fingertips with a promising future ahead and he would have made the most of every minute."

It continued: "The accused is now awaiting his sentence however long that may be, his incarceration will come to an end eventually but for Jack's family a life sentence from which there is no end is just beginning.

"We can only hope that Lord Doherty inflicts the maximum sentence possible for this violent and atrocious crime. Finally, the family ask that their privacy is respected to allow them time to try to come to terms with the tragic devastating and needless loss of Jack."

Area procurator fiscal for Lanarkshire Janet Cameron said: "Craig Roy armed himself with a knife and carried out a violent, sustained and murderous attack on Jack Frew.

"Jack Frew died because Craig Roy had a knife and had no hesitation in using it and using it repeatedly. This crime yet again demonstrates the devastating consequences that can follow from carrying a knife.

"The senseless loss of such a young life is tragic and caused shock and concern not just to the local community in East Kilbride but across our country.

"Today's conviction should act as a warning to anyone thinking of leaving home with a knife or using a knife that prosecutors, working together with the police, will ensure that those people are caught, prosecuted and brought to justice. Our thoughts remain with the family of Jack Frew at this time."

During the course of the trial Roy gave evidence where he said he "came to the conclusion" that he had repeatedly stabbed Mr Frew, but "logically" could not accept that he did.

Roy previously told the trial he thought his victim wanted to perform sexual acts with him, after claiming they had done so previously.

The killer said Mr Frew had threatened to tell his boyfriend Christopher Hannah, who also gave evidence during the 12-day trial.

Mr Hannah attended the scene of the murder near Wellesley Crescent in the town after Roy had called him sounding "erratic" following the stabbing.

He told the court that when he went to the scene, he found Roy standing near Mr Frew’s body with a knife in his hand.

• Jack Frew: Popular teenager who was 'bubbly and kind'