Students summoned the gym teacher, who radioed for help. When the teacher couldn't get through to anyone on the radio, he sent students to get the nurse, the witness said.
The other students in the class were rushed into a classroom by teachers and other staff and didn't find out until a few hours later the student had died.
"We started piecing it together," the witness said. "All the teachers were crying."
District spokeswoman Marie Reynolds said the school's crisis team, Mount Laurel police, and emergency medical technicians responded and took the student to the Virtua hospital in Marlton, where he was pronounced dead a short time later.
"It's very, very important for the community to come together and to help our young people understand what happened, that this is a rare tragedy that does happen," said Mount Laurel Mayor David D'Antonio. "My heart hurts knowing this. I have a son who's 14."
It was unclear what medical emergency caused the student to collapse.
Julia Cohen, 14, called the student one of her close friends.
"Whenever I had difficulties, he would always help. He was someone I could rely on. He's really strong," she said. "Just him saying hi to me in the hallway, it would, like, brighten up my whole day."
Cohen said the boy enjoyed wrestling and drawing and had lost a sister when he was 4.
"It was really mind-blowing," she said, "because out of all people for this to happen to, it's someone who suffered through other stuff."
Vincent Bounasissi, 14, is also in the eighth grade at Harrington and described the student as a "sweet, funny guy, always polite to the teachers."
"I'm just shocked. I just talked to the kid today, shook his hand and everything," said Bounasissi.
Parents received text messages and e-mails alerting them that a student had passed away and that counselors would be at the school Thursday to provide support to students and staff.
The eighth-grade class is set to graduate next month.
On Twitter and Instagram, classmates posted stories about the student along with photos. They spread word to wear blue - the school color - in memory of their friend and tweeted out hashtags "MTL strong" in solidarity.
"This is a good school, everybody's got each other's back, especially in situations like this," said Bounasissi.
During NJ Ask texting last week, Bounasissi said, the young man complimented a crucifix chain he was wearing around his neck. Bounasissi responded, saying he liked the chain with a turtle charm that the boy was wearing.
"He said it was in memory of his sister who passed away when he was 4," Bounasissi said. "He said he wears it every day for her."