Law enforcement and paramedics arrived to the campus after the incident was reported just after 10:30 a.m.
The Los Angeles County Fire Department said it received a call of someone who was "punctured in the side" with a pencil.
The stabbing happened when two students got into an altercation on campus, according to a statement from Valencia High.
An investigation by sheriff's deputies led to a 15-year-old boy being arrested on suspicion of assault with a deadly weapon. He was taken to Sylmar Juvenile Hall.
A sheriff's department spokesperson said the student who was stabbed, also a 15-year-old boy, is in stable condition.
It's unclear what sparked the fight. An investigation is ongoing.
A lockdown was issued while first responders were at the school. School officials later lifted the lockdown, citing no threat to the campus.
Counselors were made available to provide support to students.