Police said the stabbing appears to be the result of an argument involving the three students that had been going on for about two weeks.
The incident occurred on the fifth floor of the Urban Assembly School for Wildlife Conservation on Mohegan Avenue in East Tremont in the Bronx.
"They were harassing him and that's when he did what he did", said the friend, who would only identify herself as Tanaisha B. She said she graduated from the school in June and knew the suspect "really well".
A 16-year-old student who was stabbed in the side and chest was taken to St. Barnabas in critical condition. It was not immediately clear whether the attacker was also a student.
CBS New York reports that the school, which is in New York City's Bronx borough, was placed on lockdown after the incident, while parents gathered around the school, waiting for their children to be released.
The Urban Assembly School for Wildlife Conservation has 545 students enrolled in grades six through 12, according to the New York City Department of Education. "We are working closely with the NYPD on the investigation and providing ongoing guidance support and counseling for the school community". On Thursday, metal detectors and additional school security officers will be made available for random screenings at the school.