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Four high school students accused of beating a classmate to death in Las Vegas have been formally charged with second-degree murder.

Jonathan Lewis Jr, 17, died after a fight in an alleyway near his school, where as many as 10 of his classmates swarmed him.

Police said nine students in total have been arrested so far.

The first four to be charged are under 18 but will be tried as adults under Nevada law.

Dontral Beaver, 16, Damien Hernandez, 17, Treavion Randolph, 16, and Gianni Robinson, 17, appeared in court on Tuesday where the charges were read.

All are being held in jail.

The fight, which was caught on a video shared to social media, took place near Rancho High School on 1 November. Police said the brawl broke out over a set of headphones and a vape pen that was stolen from the victim or his friends.

Las Vegas Metropolitan Police's homicide lieutenant Jason Johansson declined to show the video to the press, saying it was "very graphic" and "devoid of humanity".

He said the video showed Jonathan taking off his shirt to prepare for the fight before he was immediately swarmed by 10 students.

Mr Johnson said the video shows the teens "kicking, punching and stomping" on the victim after he is pulled to the ground.

The victim was left unconscious and was found by a member of the public who brought him back to school. He was later taken to hospital after school staff attempted CPR on him.

The teenager died six days later after suffering "non-survivable head trauma", police said. A post-mortem examination determined his death a homicide by blunt-force trauma.

Police said they have arrested nine students, who are aged 13 to 17, and are still working to identify all those who were involved.

Those who have not yet been charged are awaiting separate court hearings because they are all under the age of 16.

Clark County District Attorney Steve Wolfson told reporters on Tuesday that his office chose to charge the teens with second-degree murder instead of first-degree murder because the evidence indicated the beating was not premeditated.

Robert Draskovich, a lawyer for one of the students charged, said he believed the videos of the fight were "incomplete".

"I'll be going through all the videos with my investigator to see what really happened," Mr Draskovich told reporters.

Under Nevada law, the accused face 10 years to life in jail.