A New York City high school and its neighborhood are reeling after a stabbing Wednesday left one student dead and another in grave condition, an incident the school's dean attributes to "What it's always about — Bullying."

A child is hugged while leaving Urban Assembly School for Wildlife Conservation, Wednesday Sept. 27, 2017, in New York.

An 18-year-old student of the school is in custody, and police say he fatally stabbed one of his fellow students — a 15-year-old boy — with a switchblade. The New York Police Department said another victim, a 16-year-old boy, was also stabbed in the arm and the torso and is in grave condition at an area hospital.

The melee unfolded about 15 minutes into third period at the Urban Assembly School for Wildlife Conservation, a public middle and high school in the Bronx, New York police said. About 1,100 students attend school in the building.

In this photo provided by the New York Police Department, a switchblade knife rests on a paper towel in New York, Wednesday, Sept. 27, 2017. Police say a high school student suddenly attacked two classmates with the knife during class at the Urban Assembly School for Wildlife Conservation on Wednesday.

"All our hearts are very heavy right now," Mayor Bill de Blasio told the media. "We will do all we can to support the school community, which is reeling right now."

It’s the first homicide on New York City school grounds since 2014, when a fight between two 14-year-old boys ended with one stabbed to death outside Intermediate School 117 in the Bronx. The last time someone was killed inside a school was at least 20 years ago.

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After Wednesday's attack, the 18-year-old suspect walked out into a hallway, where he met a counselor and handed her a bloodied switchblade, police said. He went to the assistant principal’s office and quietly sat waiting for the authorities to arrive, New York Police Chief of Detectives Robert Boyce Boyce said.

In this frame grab from video provided by WABC, police are gathered near the front door of Urban Assembly School for Wildlife Conservation in New York, Wednesday, Sept. 27, 2017. Police say a 15-year-old boy has been stabbed to death and a 16-year-old critically injured when they were attacked at the school which has middle and high school students attending.

About 15 to 20 other students were in the classroom, but no one else was injured, Boyce said.

Students described tense moments huddled in closets and in classrooms while the school was locked down, wondering what was happening.

“The guidance counselor couldn’t even keep herself calm,” said eighth-grader Abbie Mincey. The counselor told them: “I’ve never seen so much blood in my life.”

The school does not have metal detectors. The school also dismissed classes early but planned to reopen Thursday.

New York police said at a news conference that the suspect had been feuding with the victims since the beginning of the school year.

Law enforcement sources told News 4 that the teen's mother is thought to be in Puerto Rico and police have been unable to contact her to let her know about the death of her son. The U.S. territory has had spotty power and cellular service since Hurricane Maria passed over the island last week. 

Outside the school on Wednesday afternoon, frightened and angry parents gathered, demanding their children be released from inside the school building, the New York Times reported. 

As a scuffle broke out between anxious parents seeking news about their children, Kevin Sampson, the school’s dean, stood with his head in his hands. He had performed CPR on the student who died, the Times reported. Sampson told the newspaper that the three students had been locked in a running dispute over the first weeks of the school year, and that it blew up inside a fifth-floor history classroom around 10:45 a.m. in front of about 20 other students.

“Two of my students got stabbed and one of them died,” Mr. Sampson said. “It was about what it’s always about, bullying."

Giselle Estevez, the mother of 9- and 13-year-old girls, said she’d seen violence at the school before and this latest attack was the final straw. She plans to pull her daughters out of school.

“There is too much fighting, too many older boys,” she said in Spanish. “Look at my child, she is small, 9 years, and the other 13. … And they’re crying, wondering what is going on, and the school didn’t even call me.”

The last student killing inside a New York City public school was on Feb. 26, 1992, when a 16-year-old and a 17-year-old were shot dead in a Brooklyn high school by a 15-year-old, ABC News reported.

Contributing: Associated Press