Nguyen Tran Trung Kien, 17, got eight years for murder, and his eight accomplices, aged 17-20, got two to seven years.
The jury described them as "extremely dangerous" to society.
Kien was the mastermind.
In 2022, as a 10th grader at Nguyen Van Ranh High School in Tan An Town in Long An, he got into a quarrel with an unidentified 11th grader.
He messaged the boy on social media, and the two agreed to meet for a duel on Oct. 17, 2022.
But Kien showed up with eight other young men, and they immediately surrounded the 11th grader when he arrived and started hitting him with crash helmets.
They continued the brutal assault until some passers-by shouted "Police," leaving the victim unconscious on the ground.
Some others took the boy to the hospital but he died shortly afterward of brain injuries.
But at the trial the jury acknowledged that all nine defendants had expressed remorse for their crime and confessed.
Besides, their families had, on their own volition, paid the victim's family VND300 million (US$12,150) as compensation.
Saying they were mitigating factors, the jury decided to show leniency to the nine.
The police are investigating if 12 others gathered to watch the assault and could charge them with "causing public disorder."