Officers were called to San Pasqual High School around 3:15 p.m. Thursday to investigate the stabbing, according to Escondido police Lt. Bob Benton.
``There was an unknown disagreement between the two, and the suspect suddenly stabbed the victim,'' Benton said in a statement.
Witnesses saw the man drive away in a white pickup truck with a lumber rack and landscaping tools in the back, according to the lieutenant. Police described the man as Hispanic, around 20 years, with a thin build and a mustache.
The teen was taken to Palomar Medical Center. The extent of his injuries was not immediately reported.
The stabbing was being investigated by the Escondido Police Department's Crimes of Violence Unit.
Police asked anyone with information to call Detective Greg Clark at (760) 839-4450 or San Diego County Crime Stoppers at (888)580-TIPS. Tipsters can remain anonymous.