Daniel Fernandez, a 16-year-old resident of Sayreville, died Friday evening on his way to a sweet 16 party, according to Port Authority spokesman Steve Coleman.
His head then struck the underside of an overpass, he said.
The incident occurred after the New Jersey-bound bus departed Queens, New York -- loaded with 65 teenagers -- and had just crossed the George Washington Bridge.
Off-duty emergency medical technician Leon Tyrone McKivor, 52, said he approached the scene to offer his assistance and escorted worried parents to a nearby police station where their children were waiting.
"One individual had on a blood soaked shirt that he refused to take off until police insisted that he change," McKivor told CNN. "A number of other individuals had blood all over them, as well."
McKivor said he tried to console Fernandez's mother.
"She was just crying and crying and hugging me and thanking me," he said.
The horrific scene left several party-goers stunned, according to multiple posts on social media.
"Sitting here with your blood on my foot wishing this was all a bad dream," tweeted teenage party-goer Vicky Budz. "Can't sleep with you on my mind cause the more time goes on the more its settling in."
News of the incident spread quickly via the microblogging site with over 500 tweets to the #staystrongdan hashtag within a few hours of the accident.
Fernandez was brought to Hackensack Medical Center where he was pronounced dead, said Nancy Radwin, a hospital spokeswoman.
Coleman noted that the boy had suffered from severe head trauma, but would not comment further.
The bus is boasted online as "the largest custom made party bus in the country," according to Designer Transportation, which released a statement expressing sympathy for the teen's family.
"Our company cares about the well being of our patrons," the company said.