Police say it happened at 12:05 p.m. Thursday, shortly after students were dismissed from a half day of classes at the Country Club School.
The school is located at the Willingboro Board of Education Administration Building on Beverly Rancocas Road.
Officers arrived to find a 15-year-old girl who had been stabbed by another student. The attacker had fled the scene.
The victim was taken to Cooper University Hospital.
There was no immediate word on her condition, or on what sparked the violence.
Police caught up with the suspect a short time later on the grounds of the Rancocas Valley Golf Course.
That student, said to be 15 years old as well, is now in police custody.
Police confirmed that both suspect and victim are students in the Willingboro School District.