NEW YORK >> A teenager headed to a sweet 16 party was killed after he stuck his head out of the emergency hatch of a double-decker bus and hit the underside of a highway overpass, authorities said.

The gruesome accident followed the warnings of a security guard on the bus who said he told the teens repeatedly not to open the hatch.

Daniel Fernandez, 16, was among 65 teens aboard the bus Friday night from New York City on its way to the party in Garfield, N.J., said Steve Coleman, spokesman for the Port Authority of New York and New Jersey.

The teens were dancing and the bus had gotten hot, the security guard, Alex Franco, told the Daily News. Still, he said, he had asked the teens to leave the emergency hatch alone.

“I told them not to open the hatch, like three or four times, but kids, they don’t understand,” he said.

Fernandez’s English teacher at St. Francis Preparatory School in Queens echoed that sentiment.

“Sometimes kids don’t recognize their own mortality,” said Jane Lynch, her voice trembling.

Franco said he had gone downstairs to tell the driver that it was getting too hot.

“Two, five minutes I was downstairs,” he said.

But then he heard teenagers screaming, and he saw Fernandez on the floor of the bus. “There was so much blood everywhere,” he told The New York Post.

Fernandez, who lived in Queens’ Woodside neighborhood, was pronounced dead at a hospital.

He was a beloved student, said Lynch, who taught him last school year.

“He was an adorable, low-key kid, with a sweet smile,” she said. “Kids loved him; he had lots of friends and was popular with the girls.”