The sexual assault lasted a few minutes, and the suspect let the victim go.
The teen told authorities at Nogales High School what happened, and officials are worried it may happen again.
Cpl. Rudy Lopez from the West Covina Police Department advised students to travel in packs.
"The buddy system always works," Lopez said.
He also said students should keep their cell phones with them and be aware of their surroundings.
"I'm scared to walk home alone now," said student Miranda Avalos.
Authorities said they will be increasing police patrols in the area.
The suspect was described as a male with black spiky hair, medium build, about 5 feet 8 inches tall, and possibly Hispanic. He was wearing a black hooded sweatshirt at the time of the attack.
Anyone with information on the sexual assault is urged to call West Covina Police at (626) 939-8557, or the anonymous tip line at (626) 939-8688.
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