A teenage girl who stabbed a random student an Adelaide private school, as practice for something more sinister, has been spared from jail and escaped without a conviction.

The 15-year-old girl was this morning released on an eight-month supervision order despite admitting stabbing the year 12 student in August last year.

The accused, who can't be named, approached the victim in a toilet block and stabbed a knife into her shoulder blade.

Friends of the teenage schoolgirl, who planned to attack pupils who had bullied her for years, at Adelaide Youth Court. (9news)

The court was told the accused didn't know her victim and the attack was practice, as she intended to attack five other girls who'd bullied and assaulted her over a sustained period.

The teenager pleaded guilty to aggravated intentionally causing harm but a second charge of possessing an object with intent to kill was dropped.

In sentencing today, Magistrate Oliver Koehn acknowledged the girl was remorseful and battling mental health issues at the time.

The teenager picked on a random pupil in the school toilets as a trial run for her intended targets. (9news)

The girl, who spent a month in custody after her arrest, has been spared any further time in jail, with the judge also agreeing not to record a conviction.

"This has been a very difficult period in your life," he told the girl.

"I believe you've grown up a lot in the last eight months."

She's been ordered not to assault, intimidate or harass her victim or any of the five girls she allegedly intended to target.

Immediately after the stabbing the girl told her victim, "I'm sorry, I'm sorry," prosecutor Paul Tate said in submissions yesterday.

Mr Tate said upon searching the toilet block police found the girl's shoulder bag and inside they recovered a small Swiss Army knife, a black silver flick knife and a pair of scissors.

The court heard that in an interview with police the girl had said her actions and plans were motivated by the bullying and insults she had suffered for years.

Defence council Yasmin McMahon said her client had done well at school until year seven when some new students arrived.

That led to her being excluded from her friendship groups and by 2017 she was suffering significant mental health issues.

"(The girl) was experiencing significant bullying by what's termed the 'cool group'," Ms McMahon said.

"She had lost her friendships and her sense of identity in the school."