Melissa Todorovic had argued self-incriminating videotaped statements she gave to police after the death of Stefanie Rengel should not have been allowed as trial evidence.
Among other things, Todorovic argued police had not properly informed her of her rights, including the right to have a lawyer present when she gave the statements.
The Ontario Court of Appeal disagreed, saying the trial judge had properly considered the issue and found the teen had waived the rights in question.
"Interpreting what occurred between the appellant and the police officers and, importantly, whether the appellant understood her rights was a matter for the trial judge, absent a palpable and overriding error," Justice Marc Rosenberg wrote on behalf of the panel.
"I have not been persuaded that there was any such error."
Todorovic, now 22, of Toronto, was convicted in 2009 for masterminding the murder of Rengel — a girl she had never met but considered a rival.
Evidence was that the then-15-year-old Todorovic had goaded her boyfriend, David Bagshaw, 17, into killing Rengel, someone he had briefly dated two years earlier.
Bagshaw, also convicted of first-degree murder, stabbed Rengel six times and left her to die in a snowbank on New Year's Day 2008.
Shortly after the killing, Todorovic told police that the murder was more her idea than Bagshaw's, but that she did not believe he would take her seriously.
She also admitted knowing Bagshaw was headed to Rengel's house that night to kill her and that she called him 15 minutes after the murder to ask: "Is she dead?"
Todorovic also told police that she had asked Bagshaw to come over to her house to show how he had killed the girl.
Those statements, her lawyer had argued, contributed to a miscarriage of justice.
For its part, the Crown had argued any rights breach was minor and the evidence against Todorovic — including reams of messages between her and Bagshaw in which she threatens to break up with him or withhold sex until he killed Rengel —was overwhelming.
Todorovic was sentenced as an adult to life in prison with no chance of parole for seven years — the maximum adult sentence for someone her age.
The Appeal Court, which heard the case in November, also dismissed her arguments she should have been sentenced as a youth.
Todorovic showed no empathy for Rengel and harboured ongoing thoughts of hurting others," the Appeal Court noted.
Bagshaw is also serving an adult life sentence after pleading guilty to first-degree murder.