Montclair police say the attack happened in the 4700 block of San Bernardino Street at around 3:34 p.m. when the assailant approached the girl from behind and stabbed her in the head with a "fixed bladed knife."
The victim turned around and was stabbed several more times by the suspect, police said.
Witnesses stopped the attack and detained the boy until police arrived.
The girl was taken to a hospital to be treated for her injuries that were described as non-life threatening. She is expected to recover.
The girl was walking home from school before the stabbing. Police say she and the suspect both attend Montclair High School.
The teen boy was arrested and booked at San Bernardino County Juvenile for attempted murder, police say.
The investigation is ongoing. Anyone with information is asked to contact the Montclair Police Department.