Michael Hernandez, 22, was found guilty and sentenced to life in prison for the February 2004 fatal stabbing of Jaime Gough inside a bathroom at Southwood Middle in Palmetto Bay when they were both 14, The Miami Herald reported.
Wednesday's ruling tossing out Hernandez's sentence came after the U.S. Supreme Court's decision to ban automatic life sentences without the possibility of parole for juveniles convicted of murder.
Hernandez will be resentenced by a Miami-Dade trial court.
Jorge Gough, the victim's father, said the court's decision was frustrating, but his family was trying to stay positive.
"We're going to continue to trust in the God that brought us all this way," he said. "I just want to hold on to this peace that we got learning to forgive, to let go of that anger and that hate. I don't want to go back there."