Four Las Vegas teens who beat their high school classmate to death avoided lengthy prison sentences for murder after reaching a plea agreement.

Jonathan Lewis Jr., 17, died in hospital days after he was punched, kicked and knocked unconscious in an alley last November.

His killers, who were 16 and 17 at the time of the attack, were to have been tried in adult court for second-degree murder but pleaded guilty to manslaughter in a plea agreement that will see them placed in juvenile detention indefinitely.

The victim’s mother, Melissa Ready, told local media: “There is literally no one being held accountable for the murder of my son. It’s disgusting.”

She told the Las Vegas Review-Journal that she opposed the deal and was not told about it until August, when it closed.

“They knew when they stomped on my child’s head that he was going to die. They should be held accountable as adults,” she said. “They made a mature choice.”

The killers, Treavion Randolph, Dontral Beaver, Gianni Robinson — all now 17 — and Damien Hernandez, who turned 18 since the attack, were charged in January.

Nevada can prosecute children as young as 13 for murder. However, those found guilty in the juvenile system in Clark County, which includes Las Vegas, can be held in juvenile detention until they are 21.

They may be able to be released on probation early if they complete a rehabilitation program.

District Attorney Steve Wolfson defended the settlement, saying it balanced the glaring facts of the case with potential legal challenges if it went to trial.

Mr Wolfson added that the juvenile detention system would provide the killers with more appropriate means of rehabilitation.

Jonathan was fatally wounded in an alley near Rancho High School on November 1. Footage surfaced on social media showing him taking off his shirt in preparation for a fight, reportedly over stolen headphones and a vape pen.

He was then surrounded by 10 students, pulled to the ground and knocked unconscious.

A member of the public found him and brought him back to the school, where staff attempted to revive him before he was taken to hospital. He died six days later from “non-survivable head trauma,” police said.

Nine students, aged 13 to 17, were arrested over the attack.

Karen Connolly, Hernandez’s attorney, told CBS News: “Damien deeply regrets his participation in the turmoil that led to Jonathan’s tragic death.

“He accepts full responsibility and will accept any punishment imposed on him.”