A "high-achieving" student who plotted with a friend to murder a teacher at a Perth high school has been sentenced to 11 months in juvenile detention.
The girl, who cannot be identified, was 13 years old when she and her friend, another female student who was then aged 14, used social media last year to discuss killing the female teacher at Willetton Senior High School.
The Perth Children's Court was told the pair initially talked about burning down the school, but when that idea was ruled out, the 13-year-old sent a message saying "the stabbing idea should take over now".
The older girl replied by saying she was going to do "research" before detailing in later messages where and how to stab the teacher in a way that would sever an artery and cause "immediate death".
Then on November 1 last year, the friend brought a knife to school, which the 13-year-old concealed in her bag, before handing it back to her during recess.
The friend then went to a meeting with the teacher where she told the woman she had a "good surprise" for her, before lunging at her.
Police alerted to girl's role
Around the same time, other friends of the girl, who had been told what she planned to do, raised the alarm and a staff member tried to alert the teacher to what was happening.
Because the teacher got up at the same time as the girl lunged at her, she ended up suffering only a minor wound near her armpit.
The school went into immediate lockdown, but the 13-year-old girl remained silent until later in the day, when police were made aware of her role in the crime.
She was originally charged with attempted murder, but prosecutors accepted her guilty plea to the lesser count of "with intent, doing an act that caused bodily harm".
During a hearing in July, the 13-year-old's lawyers argued for the girl to be given a community-based sentence, describing her as a "high-achieving student" who accepted unequivocally that what she had done was wrong.
They also said most of the detail about what was planned had come from the friend.
But prosecutors called for the teenager to receive an immediate term of detention, and because Children's Court president Hylton Quail wanted to consider that possibility, he remanded her in custody to the Banksia Hill juvenile detention facility.
Today the girl, who is now 14, was sentenced to 11 months in juvenile detention, but she may be released after serving half of that. She has been in custody since July 21.
Teacher could have died: Judge
In sentencing, Judge Quail said it was the second time this year he has had to sentence a young person in the context of violence against teachers, though in the earlier situation the incident was prevented.
"This offence is one where general deterrence is important," he told the court.
Judge Quail described the crime as "shocking", and "completely out of character", saying the girl's family was "devastated".
"Your encouragement and support [of the other teenager was] important," the judge told the school girl, and when there were discussions about the plans, "you told no-one".
Judge Quail said if the teacher wasn't moving when the stabbing occurred, she may have died.
The community needed to know there were serious consequences for such behaviour.
Teacher still suffering after attack
The judge described the ongoing impact on the teacher as "substantial", with "profound consequences" as she suffered PTSD and had cause to question her career and identity.
He did accept the teenager was sorry for what she had done, and was unlikely to reoffend.
The court heard that since the girl was put into detention in July, there had been several lockdowns at Banksia Hill due to staff shortages and "operational demands".
Judge Quail said the sentence needed to be reduced, due to the lockdowns, which he said had resulted in her being subjected to a form of solitary confinement and "arbitrary punishment".
"Lengthy lockdowns are unlawful," he told the court.
The other teenager has pleaded guilty to a charge of attempted murder.
She has been told she will receive a term of detention when she is sentenced later this month.
