A teenager stabbed his ex-girlfriend repeatedly in the neck in a "frenzied attack" before trying to make it appear her wounds were self-inflicted.
Thomas Griffiths admitted murdering Ellie Gould, 17, at her home in Calne, Wiltshire, in May, after she ended their relationship.
Griffiths, now aged 18, went to the schoolgirl's home, killed her and then left her hand on the knife handle.
He was jailed for a minimum of 12 and a half years at Bristol Crown Court.
Carole Gould said there was nothing in Griffiths' behaviour before her daughter's death that "would ring alarm bells".
"We welcomed him into our home. We ate dinner with him," she said.
The packed courtroom heard the night before Griffiths murdered her, Ellie had told friends they had broken up and he had "not taken it well".
The pair were A-level students at Hardenhuish School in Chippenham, had known each other since Year 7, and been in a relationship for three months.
Griffiths walked out of school on the morning of 3 May and drove to Ellie's home in Springfield Drive.
There he attempted to strangle her, before stabbing her 13 times in the neck with a knife taken from the kitchen.
"Griffiths became angry, perhaps by Ellie's continued rejection of him, and he attacked her," prosecutor Richard Smith QC said.
A statement was read out in court from Ellie's father, Matt Gould, who found her lying on the kitchen floor with the knife still in her neck.
He said it was "the most frightening, horrific and saddest scene I have ever experienced" and it "fills my thoughts all day".
Evidence suggested Griffiths had put Ellie's hand on the weapon to make it look like she had done it to herself.
The court heard Griffiths spent an hour at the house before he drove home, changed his clothes and dumped a bag of items taken from Ellie's house in a wood.
Later that day he sent a series of "fake" messages to friends and to Ellie's mobile phone asking if she wanted to meet.
Griffiths also told friend marks on his neck were caused by self-harm but the court heard they most likely caused by his "young victim fighting for her life".
Sentencing him, Judge Mr Justice Garnham told Griffiths his actions had been a "frenzied knife attack" and "the most appalling act" on a "vulnerable young woman in her own home where she should have been safe".
He said Ellie had "tried desperately to fight back, scratching frantically at your neck" and "most chilling is that you left her on the kitchen floor with the knife still in her neck and with her left hand on the knife".
The judge told Griffiths it was one of several steps he had taken to "cover your tracks".
"There can be no more dreadful scene for any parent to contemplate than that which confronted Ellie's father when he came home that day from work," Mr Justice Garnham said.
The court had previously heard Ellie was a keen horse rider who competed in local shows and cross-country events, and talked of joining the mounted police.
The judge told Griffiths: "The effects of your actions have not only snuffed out the life of this talented girl... but loaded pain on her friends and family."
The court was told that following his guilty plea in August, Griffiths, of Derry Hill, Wiltshire, had written a letter outlining his "heartfelt remorse".
In it, he said: "I feel confused and angry at myself that I was able to hurt someone so special to me."
Det Ch Insp Jim Taylor of Wiltshire Police said Griffiths ended Ellie's life "in the cruellest way imaginable" and "destroyed the lives of those who were close to her".
"While I know that this prison sentence will not bring Ellie back, and 12 and a half years no doubt seems insignificant given the severity of this crime and the colossal loss for this family, I hope that in some way it provides them with some form of closure," he added.