Authorities said a fight broke out between the two students outside of a Passaic Avenue apartment building across from Passaic High School around 7:30 a.m.
One stabbed the other at least 4 times in the arm, ribs and lung, according to school officials.
The students are 16 and 17 years old.
Superintendent Robert Holster told Eyewitness News that the two boys know each other, but he doesn't know what sparked what he described as "a very aggressive assault."
Police took the suspect into custody at the school.
The victim was taken to St. Joseph's Regional Medical Center where he was listed in guarded condition, officials said.