School officials said classes and services were temporarily closed at Mt. SAC, but were set to resume at 4:30 p.m.
The threat was reported to occur at Mt. SAC in the 1100 block of N. Grand Avenue around 2:45 p.m., officials said in an emergency alert sent to students via text message.
Students were asked to calmly leave campus as a precaution.
Lt. John Saleeby of the sheriff's Walnut Station said a bomb threat was phoned in to Walnut High School around 10:24 a.m. The high school, which is located in the 400 block of Pierre Road, was placed on lockdown and sheriff's officials conducted a security sweep.
Around 1:30 p.m., the sweep concluded and the lockdown was lifted.
Saleeby said Mt. SAC decided to close the campus on their own.