A Long Island teenager was charged with murder Wednesday in the stabbing death of an Oceanside High School student who was killed while dozens of bystanders filmed the assault — without lifting a finger.

Tyler Flach, 18, of Lido Beach, is due to be arraigned on a second-degree murder charge in First District Court in Hempstead on Thursday, Nassau County police said.

Flach was allegedly one of about eight teens who attacked Khaseen Morris, 16, and his friends at a strip mall near the Oceanside school on Monday — apparently over a girl.

A second teen suffered a broken arm in the attack.

Morris had only moved into the community this summer.

His sister, Keyanna Morris, 30, said her little brother had walked a girl home from a party, making him the target of a jealous ex-boyfriend from nearby Long Beach.

“He was so happy to go to school,” she said. “He was just cheesing. He came home and he’s like, ‘I made so many friends. Everybody loves me.’ “