Gary Roper, 18, from Ilford, east London, hit Triston Christmas so hard he fell and hit his head on concrete, dying a week later, in January 2005.

Triston's mother said a mobile phone video recording of him as he lay dying had been passed around his classmates.

Roper was convicted of manslaughter, a charge he denied, and sentenced to 30 months' youth detention.

I recognise that any sentence I impose will not bring Tristan back or give any real consolation to his family Judge Anthony Morris

Prosecutor Robin Grey said Roper, who had a history of violence, appeared to have borne a grudge against Triston, who was accused of pinching his girlfriend's bottom.

Old Bailey judge Anthony Morris said the attack had been "gratuitous, unprovoked and pre-meditated".

He told Roper: "This is truly a tragic case, both for you and Triston's family.

"I recognise that any sentence I impose will not bring Triston back or give any real consolation to his family."

Triston's mother, Siobhan, left the court in tears after the sentencing.

It's absolutely heartbreaking and it will haunt me for the rest of my days Triston's mother Siobhan

She said the mobile phone video had been passed around her son's friends and had been sent to her husband Peter's phone.

"Peter got it off one of Triston's friends, but I could not look at it for more than a second. It was just too devastating to look at," she said.

"Tristan was lying there with blood coming out of his mouth and his ear and he was trying to speak.

"It sounded like he was trying to say `Mum, Mum', but he couldn't get the word out properly.

"It's absolutely heartbreaking and it will haunt me for the rest of my days."

Police said they were aware of reports that someone recorded a video phone message of Triston as he lay dying, but had not seen it.

A Scotland Yard spokeswoman said they had not received an official complaint about it, but would investigate it further if they did.