The attack sparked a debate about security inside the school in the East Tremont section of the Bronx. Thursday morning before students arrived, metal detectors arrived at the school.
The teenage suspect, Abel Cedeno, is expected to face a judge Thursday. He is accused of stabbing and killing a fellow student in a classroom, and attempting to kill another. He is charged with murder and attempted murder, manslaughter and attempted manslaughter, assault and criminal possession of a weapon.
Cedeno mouthed the word "yes" when asked Wednesday night if he had been bullied, relentless taunting that cops say led to rage.
WATCH: Cadeno being escorted to a waiting police car
Police say Cedeno stabbed not one but two teens in his third period history class at the Urban Assembly School for Wildlife Conservation and calmly walked right out.
"He was confronted by a counselor in the hallway, where she asked for the knife and recovered the knife. He then walked into the assistant principal's office and sat there and the police were called," said Robert Boyce, Chief of Detectives, NYPD.
SCENE PHOTOS: Double stabbing at NYC school
"And the next thing you know they were rushing this kid to an ambulance on a stretcher, running with him," a witness said.
Here's a look at the knife recovered:
Family members identified 15-year-old Matthew McCree as the teen who died.
The other teen injured, who is 16 years old, is in critical condition.
As detectives questioned Cedeno, his family friend told Eyewitness News that the 18-year-old had been the subject of weeks of taunts and bullying by the two victims.
"He usually gets bullied a lot," a friend said.
Earlier, friends of Cedeno's told a similar story.
"I guess he did that kind of to scare them off, not to actually hurt anyone," she said.
"He's easily threatened," another friend said. "He's mellow, he's a good person, but I guess in that situation he probably felt threatened."
But it's the family of Matthew McCree that's paying the price. They held one another and sobbed at their loss, as Cedeno headed to central booking charged with murder.
The school building did not have metal detectors at the time of the stabbing, but the mayor said there would be random checks with metal detectors starting Thursday.
The building houses elementary school P.S. 67, as well as in the Urban Assembly School for Wildlife Conservation, which serves 6th through 12th graders.
Schools Chancellor Carmen Farina said parents have a right to expect their kids will come home safely at the end of the school day. A letter was sent home to parents, and extra grief counselors will be available Thursday.