Teenage murder mastermind Melissa Todorovic is appealing her 2009 conviction just 13 months before she could be freed on parole anyway.

Todorovic's lawyer was in the Ontario Court of Appeal to argue the young Toronto woman deserves a new trial or to have her sentence changed from an adult to a youth term, The Canadian Press reports.

Brian Snell says Todorovic was convicted using improperly-obtained incriminating statements made to police before she was formally arrested and advised of her rights.

In a case that shocked Canadians for its cold-bloodedness, Todorovic was found guilty of first-degree murder in 2009 for pressuring her 17-year-old boyfriend, David Bagshaw, to kill Stephanie Rengel.

Todorovic, 15 at the time, had never met 14-year-old Rengel, but perceived her as a rival because she and Bagshaw had once dated and she believed Rengel was spreading rumors about her.

Rengel was stabbed to death on New Year's Day 2008 and left to die in a snowbank steps from her front door.

At her trial, the court heard evidence Todorovic spent months nagging Bagshaw via text messages and threatening to withhold sex to get him to kill Rengel. The day of the killing she demanded he come over and show her how he did it.

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Bagshaw, who turned 18 days after the killing, pleaded guilty to first-degree murder as a young offender but received an adult life sentence with no parole eligibility for 10 years.

Todorovic was also sentenced as a young offender and received a seven-year sentence, starting in a youth-detention facility, the maximum adult term for someone her age. A youth sentence would have meant six years in detention and four years of community supervision, CP said.

She was transferred to an adult prison in 2011 when she turned 20, though she went to court then in a bid to stay in the youth jail, where she was the oldest female inmate, CP reported at the time.

In this week's appeal, Snell is challenging Todorovic's initial interview with Toronto police investigators. According to CP, Todorovic told the two detectives that killing Rengel was her idea, that she knew Bagshaw was going to the girl's home and that she called him 15 minutes after the murder to as "is she dead?" Then she asked Bagshaw to come to her house to demonstrate just how he'd killed Rengel.

Todorovic also apparently tried to suggest it was a big joke, telling the detectives she didn't think Bagshaw would take her seriously and go through with the murder.

However, the officers did take her seriously and placed her under arrest.

Snell argued that before beginning the interview the officers should have told Todorovic she did not have to give a statement and that she could have a lawyer present if she did, CP said.

He doesn't accept that the teen was not a suspect at the time and was being interviewed as a potential witness against Bagshaw. Snell said police already knew Todorovic had been pressuring Bagshaw to kill Rengel and about the post-murder phone call.

[ Related: Young killer J.R. complying with conditions, court hears ]

Todorovic effectively was being detained during the interview and therefore should have been advised of her rights under the Youth Criminal Justice Act, Snell argued.

"Detectives testified emphatically that when they were taking the first statement they had no idea that the appellant might implicate herself in the homicide," Snell argued in his written submission to the Appeal Court, CP said.

"[But] it strains belief that two experienced homicide detectives would have approached the case with such naivety."

Snell wants Todorovic to have a new trial with the incriminating statements excluded as evidence.

The Crown is arguing the trial judge was right to allow Todorovic's statements into evidence.

"If the trial judge erred by admitting either or both of the appellant's statements, then such error occasioned no substantial wrong or miscarriage of justice," the Crown said in its arguments.

"There is no reasonable possibility that the verdict would have been different."

The Crown also contends the volume of her text messages to Bagshaw pushing him to kill Rengel were powerful evidence of her guilt.

QMI Agency also reported Snell would argue that Todorovic's messages were misinterpreted and taken out of context.

Todorovic is eligible for parole late next year.