The 17-year-old, who cannot be named for legal reasons, plunged a blade into Connor Cowper, 18, at a party at a flat in Holytown, Lanarkshire, in April 2018.
The High Court in Glasgow heard how the girl was pregnant at the time she took Connor’s life – she has since given birth while on remand at Cornton Vale prison in Stirlingshire.
The jury heard claims how Connor had “fancied” the girl but she kissed another young man that night. The Crown claimed she “loved being the centre of attention” and was trying to play the two men off against each other.
But the girl, who is thought to be among Scotland’s youngest female murderers, exploded when Connor branded her a “stupid wee girl”.
She grabbed a blade and struck defenceless Connor being fleeing the scene. The girl, who was expelled from school aged 11, later said she wanted him dead.
Yesterday at the High Court in Edinburgh, judge Lord Arthurson ordered the girl to serve at least 16 years in custody.
She was convicted last month at Glasgow High Court and sentence had been deferred for background and character reports.
Lord Arthurson had been told by defence advocate Tony Graham QC that the girl had “acknowledged” how her actions had caused “devastation” and had deprived Connor’s family of their loved one.
But the judge said he had doubts about whether this was correct.
He added: “I have been told that not only do you feel regret but you also expressed empathy towards the victim.
“However, as the presiding judge at your trial I saw nothing of that.
“Your demeanour was flat and disinterested and you didn’t give out any significant appearance of empathy.
“You are a highly dangerous and unstable individual. The attack on Connor was deliberate, brutal, entirely unprovoked and entirely murderous.”
During proceedings last year, the girl denied murder. She claimed she stabbed Connor because she feared he was going to hit her.
She also claimed to have been affected by a previous abusive relationship at the time of the fatal assault.
The court heard how the girl had only known Connor through Facebook before the party.
She told jurors that Connor was a “nice person”.