A student was later taken into custody, according to the Los Angeles County Sheriff's Department.
Lone Hill Middle School and San Dimas High School were placed on lockdown around 9:30 a.m. as authorities searched the high school, where the caller claimed the armed person was seen walking.
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Sheriff's officials said the claim, which came from an anonymous caller, was unfounded and no injuries were reported.
The Bonita Unified School District tweeted around 10:30 a.m. that authorities believed the report was from a prank call, but the threat was still investigated.
AIR7 HD was over San Dimas High where dozens of parents and students were seen outside of the school.
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The school district tweeted around 11:15 a.m. that the lockdowns had been lifted at both schools.
"We have confirmed to a satisfactory level that the call was a hoax, and that the call came from a student," the Sheriff's Department said in a statement. " We are prohibited by law from identifying juveniles arrested or detained, so we will be unable to do so in this case."