Tyler Flach, 18, is accused of second-degree murder in the deadly after-school brawl that broke out not far from the victim's Long Island school.
Police said that most of the "50, 60, 70" kids who witnessed the attack on Khaseen Morris did nothing to stop it.
"That's egregious," said Detective Lt Stephen Fitzpatrick.
"Kids stood here and didn't help Khaseen," Lt Fitzpatrick said. "They would rather video this event."
Police were called to a row of shops near Oceanside High School in Nassau County on Monday afternoon.
Mr Fitzpatrick said the fight, which took place outside a pizzeria, was a "pre-arranged dispute" over a girl.
Police say the main suspect and six or seven other assailants charged at Khaseen, who was unarmed.
The 16-year-old was taken to a local hospital where he died overnight from a single stab wound.
A second victim, a 17-year-old boy, was in hospital with swelling to his head and a broken arm.
According to authorities, many of the bystanders recorded Morris' assault, posting footage to Snapchat and other social media platforms, but none intervened.
"They videoed his death instead of helping him," Mr Fitzpatrick said of the teens looking on.
Photo Tyler Flach has been charged with second-degree murder