A teenaged girl has been sentenced to 30 months in a youth detention centre after pleading guilty to the killing of a 17-year-old boy on Nuns' Island.

The girl and her boyfriend pleaded guilty this week to manslaughter and conspiracy to commit armed robbery.

The girl was sentenced as a youth, but the other killer may be sentenced as an adult. A hearing to determine whether he will be sentenced as an adult or a minor takes place on May 14.

The killing took place last November in a wooded area of Nuns' Island known as Domaine Saint-Paul. The killers were planning to steal an ounce of marijuana from the victim, who had recently begun selling the drug.

According to the Crown prosecutor, the couple plotted by text for the girl to pretend to seduce their victim in the woods only for her boyfriend to attack.

In court the boy admitted to slashing the victim across the thigh, a deep cut that lead to the victim bleeding to death. The victim's body was discovered the next morning.