Update 5:30 p.m: The lockdown has now been removed with no gun or other suspicious activity found. The student release will be done slowly, classroom by classroom.
Update 5 p.m: Back to School Night has now been canceled.
Update 4:30 p.m: Arcadia High's Back to School Night is still scheduled to take place tonight, according to a tweet from Arcadia Unified.
Update: A student is being medically evacuated, according to Sgt. Tom La Veque of the Arcadia Police Department, but the medical issue is not related to the threat. The E. SGV Scanner Twitter feed notes that scanner traffic indicates it is an asthma attack.
Original: Arcadia High School is on lockdown because of an anonymous phone-call of a man with a gun, according to a message sent out by the Arcadia Police Department.
The students are being directed to appropriate locations inside the school and officers are on scene, according to the mesage. There have been no suspects or suspicious persons seen so far.
Parents are directed to wait for students in the east parking lot of the County Park near Santa Anita and Diamond, Arcadia. Students are not yet being released.