A stabbing at P.S. 67 left one student dead and another critically injured Wednesday in the middle of third period, police say.

Abel Cedeno, 18, allegedly stabbed two classmates in the chest with a knife in a fifth-floor classroom at the Urban Assembly School for Wildlife Conservation, according to Robert Boyce, the NYPD chief of detectives.

The three students had been arguing for several days prior, he says, and the attack may have come in response to bullying.

First responders brought both victims to St. Barnabas Hospital, where 15-year-old Matthew McCree died. Police say 16-year-old Ariane Laboy, the second victim, remains there in critical condition.

After the attack, police say Cedeno walked out of the classroom and surrendered the weapon to a school counselor. He was taken into custody.

Education officials say it's the first deadly act of violence in a public school in 25 years.

City Schools Chancellor Carmen Farina says the school is cooperating with police in the investigation. She condemned the violence alongside Mayor Bill de Blasio and police officials Wednesday.

"I'm heartbroken at the loss of one of our students, and this tragedy should never have taken place," she said.

Officials say they'll begin searching students entering the school, which has no metal detectors.