The non-credible threats were made against Ayala High School in Chino Hills and Upland High School.
In the wake of Friday's massacre at Sandy Hook Elementary School in Newtown, Conn., authorities said they have taken even the slightest hint of a threat seriously in recent days.
Police officers did not detail the nature of the threats or how they were made.
"We're not taking any threat lightly," Upland police spokesman Lt. Eleno Arriaga said.
"There is no evidence that the rumors are true at this time."
A Upland High student was investigated for allegedly making the possible threats on the campus.
"The parents of the student were extremely cooperative and allowed us to search their home for evidence," Arriaga said.
The Upland Unified School District had emergency lockdown procedures in place even before Friday's mass murder, which led to the death of 20 elementary-aged children and six others at the school, Superintendent Gary Rutherford said. Despite the procedures, rumors swirled on Twitter and other social media sites.
"It's a very routine day except for the social media activity," Rutherford said.
"We're safe and sound but we're not taking any chances."
The alleged threats against Ayala High started after a parent passed along a rumor that she had heard, said Julie Gobin, Chino Valley Unified School District spokeswoman.
"We have a parent who allegedly heard something, but has elected not to share the information with the (San Bernardino County) Sheriff's Department," Gobin said.
"The (security) officer is investigating, but, at this time, we don't have the information that would allow authorities to say whether it's credible or not."
Deputies were investigating what they said were rumors related to the upcoming Mayan apocalypse, sheriff's spokeswoman Cindy Bachman said.
"There are no specific threats at this time," Bachman said.
The threat at Upland High also made reference to the Mayan calendar, which predicts the end of the world will come on Friday.
"Apparently, it's a combination of what just happened in Newtown and the end of the Mayan calendar," Rutherford said.
Access and egress points to the campuses at Upland and Ayala high schools were sealed except for their main entrances. They both also have an electronic switch that implements lockdown procedures and turns on surveillance cameras.
Teachers at Chino Hills High had complained in the days following the shooting that locks on their classroom doors weren't working at a meeting that was set to review security procedures at the school.
The district is looking into the situation, Gobin said.
Gobin said it's not unheard of for students to try to call off school with a phony threat.
"What might not get heard or remembered on other days is something that has people more nervous now," she said.
"We do take every rumor seriously."
Both districts said they would keep a heightened level of vigilance this week.
"We will aggressively prosecute people making threats or hoax threats for the disruption that it creates during these highly sensitive times," Rutherford said.
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