Updated

A high school student attacked two classmates inside a school classroom Wednesday, fatally stabbing a 15-year-old boy and critically injuring a 16-year-old boy, police said.

The melee unfurled around 10 a.m. on the fifth floor of the Urban Assembly School for Wildlife Conservation, a middle and high school in the Bronx.

Angry parents, some in tears, gathered outside the school demanding they be allowed to pick up their children. Police and emergency crews swarmed the school, located in a large red brick building that also houses a public elementary school.

Rita Taylor, the mother of a 5-year-old student, screamed at authorities to let the children out of the school.

"I'm not going to calm down!" Taylor shouted, according to the Daily News. "My daughter is in there crying! I know my child!"

An 18-year-old man was taken into custody, and a weapon was recovered, investigators said. Police weren't certain what prompted the stabbing but were looking into whether he had been bullied.

The 15-year-old was stabbed in the chest and died. The 16-year-old, who was gravely injured, was stabbed in the side and the chest.

The school doesn't require students to pass through metal detectors to enter the building. Authorities did not immediately say what type of cutting device was used.

Schools Chancellor Carmen Farina says she is heartbroken by the violence.

"This tragedy should never have taken place," she said.

It's the first homicide at a New York City school since 2014, when a fight between two 14-year-old boys ended with one stabbed to death outside Intermediate School 117 in the Bronx.

New York has the nation's largest school district, with more than 1 million students.