A once "high-achieving" Perth student is behind bars while the President of Western Australia's Children's Court decides whether she should receive a term of immediate juvenile detention for her part in a plot to fatally stab one of her teachers at Willetton Senior High School.

The girl, who cannot be identified, was only 13 years old last year when she and a friend discussed the plan, over a period of two weeks, on the social media app Discord.

Their messaging was detailed during a sentencing hearing, with the Children's Court told the two had first talked about burning down the school.

However, after concluding that plan would not be successful, the 13-year-old sent her friend a message saying: "The stabbing idea should take over now."

The friend replied by saying she was going to do some "research", with later messages going into detail about how and where to stab the teacher in a way that would sever an artery and cause "immediate death".

The court was told that, on November 1 last year, the friend messaged the 13-year-old to say she had "acquired the knife" before the two rode to school together.

Because the friend was subject to bag searches, she gave the knife to the 13-year-old, who hid it in her bag for a couple of hours, before she handed it back.

Heroic students

The friend then went to the teacher's office and, during a meeting, she lunged at the teacher and tried to stab her, with the intention of killing her.

Fortunately, the woman got up to answer a knock on the door at the crucial moment, meaning she ended up suffering only a 1-centimetre wound around the area of her armpit.

Prosecutor Brad Hollingsworth revealed the knock on the door was the result of three other students, who had been told that morning about the stabbing plan, raising the alarm.

"They are the ones who, potentially, saved the teacher's life," he told Judge Hylton Quail.

The school went into immediate lockdown, but the court heard the 13-year-old said nothing until police were made aware of her involvement in the crime.

While her friend was originally charged with attempted murder, prosecutors accepted her plea of guilty to the lesser offence of "with intent, doing an act that caused bodily harm".

Her lawyers submitted that the teenager, who is now aged 14, should receive a community-based sentence.

Barrister Simon Watters, who is representing the 13-year-old, described his client as "a high-achieving student", saying she accepted "unequivocally" that it was wrong to hide the knife and to be involved in the crime.

Mr Watters said most of the detail about what was planned had come from the friend, with his client sometimes replying with statements such as "WTF". She also refused the friend's request for her to lend a knife to her, when she said she was initially unable to "access" one on "the planned date".

He maintained that, despite the two weeks of messages, the stabbing plan had only really "crystallised" on the morning it happened.

The prosecution submitted that Judge Quail should consider imposing a term of immediate juvenile detention on the 13-year-old, with Mr Hollingsworth describing the crime as "chilling", "deliberate", "disturbing" and "frightening".

He said the teenager had hidden the knife for her co-offender, knowing that it was going to be used to endanger the life, health or safety of the teacher and that neither before, nor after, the stabbing — when the school was in lockdown — had she said or done anything to let people know about their "dastardly plan".

Mr Hollingsworth urged the judge to impose a sentence that would help protect teachers and send a message to the community "that such deliberate and callous acts of violence will be punished severely … even for first-time offenders".

Banksia Hill worries

Judge Quail said he needed time to consider what sentence to impose on the 13-year-old girl for a number of reasons, including what he called "the state of crisis" at the Banksia Hill Juvenile Detention Centre.

He said that, because of staff shortages and lockdowns at the facility, he could not be satisfied that any juvenile who was sentenced to a term of detention would not just be locked up in their cell, day after day, and not have access to schooling, rehabilitation programs or family visits.

While he did remand the teenager in custody, he said he would receive a report on her experience at Banksia Hill and whether he could "safely" conclude that detention was the "right" penalty for her.

As the judge revealed that the teenager would be detained, her mother, who was sitting in the public gallery with the girl's father, broke down in tears.

The 13-year-old girl will now be sentenced in September, after her co-offender, who has pleaded guilty to the more serious charge of attempted murder, is dealt with.