The girl, who cannot be named for legal reasons, was previously found guilty of attempting to murder teachers Fiona Elias and Liz Hopkin and a pupil at Ysgol Dyffryn Aman in Ammanford.
Emergency services were called to the school on 24 April last year, in what the trial heard was a “serious episode of violence” after the girl took her father’s fishing “multi-tool” to school.
She had admitted to three counts of wounding with intent and possession of a bladed article on a school premises, but a jury found her guilty of attempted murder in February after a week-long trial.
Following her arrest, the teenager told officers she was “pretty sure” the incident would be on the news, and added “that’s one way to be a celebrity”.
Both Mrs Elias and Mrs Hopkin “received significant and serious injuries”, Swansea Crown Court heard.
Mrs Hopkin was airlifted to the University Hospital of Wales in Cardiff after she sustained “four stab wounds”, while Mrs Elias and the pupil also attended hospital for treatment.
Handing down his sentence, Judge Paul Thomas said the defendant would serve half of the 15-year sentence before she can be considered for release.
Addressing the defendant, he said: “What you did in school almost a year ago the day has caused a large number of people a great deal of harm and upset. It has hugely affected many lives, including, of course, your own.”
“The simple fact is you tried to kill three people, two teachers and another pupil,” he added.
“I think that it is very important here that what you did you did in full of so many other pupils…In my view you wanted as many of your fellow pupils as possible to see what you intended to do.”
“For one reason or other, you weren’t really listening” to the victim impact statements of Mrs Elias and Mrs Hopkin, the Judge added.
He said he did not think the teenager was “genuinely sorry” for what she did, adding: “You showed no emption or even interest in how they felt that day or ever since.”
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