A Long Island college student was arraigned Thursday in the stabbing death of an Oceanside high school student whose vicious attack was caught on video by do-nothing bystanders.

Murder suspect Tyler Flach, 18, of Lido Beach, pleaded not guilty to second-degree murder as the family of 16-year-old stabbing victim Khaseen Morris — who filled three rows in the packed Hempstead courtroom — looked on in grief.

Flach, a freshman at Nassau Community College, was ordered held without bail.

He was allegedly part of a mob that ambushed Morris and his friends Monday in a strip mall near Oceanside High School — reportedly over a girl.

Police are seeking at least seven other suspects, but said it was Flach who stabbed Morris once in the chest. The victim was rushed to an area hospital, where he was pronounced dead.

The college student surrendered on Wednesday after video of the brutal attack began to go viral on social media, with witnesses posting it to Snapchat, Facebook and Twitter, among other sites.

“This is the very, very beginning of a very serious case,” said Edward Sapone, Flach’s lawyer. “The people involved are all young, they’re all students. There are many sets of families that are suffering.”

Sapone said his client has received death threats since his arrest, and asked that the teen be held in protective custody.

The lawyer drew gasps from Morris’ friends and family when he suggested in court that some of the slain teen’s friends could be affiliated with the Bloods gang.

“I’m not alleging that he is but there is certainly evidence on social media that tends to suggest that that group is involved with street gang violence,” Sapone told reporters later.

“There are gang signs, there is hair that is dyed blood orange or red,” he added. “There are guns on social media and so there is certainly evidence that that group has those connections.”

Nassau Detective Lt. Stephen Fitzpatrick said detectives were unable to interview Flach after he surrendered because “his attorney asked us not to.”

Morris was reportedly attacked over a girl he had walked home from a party. The unidentified girl’s ex-boyfriend became jealous and showed up at the strip mall to get Morris, police said.

Sapone said Flach was a friend of the ex-boyfriend, who was not identified.