Police say the threat was a hoax.
South Otago High School was evacuated at lunchtime after the office received a call about 12.30pm saying there was a bomb on the school grounds.
Sergant Martin Bull, of Balcutha, said pupils were moved to a safe muster area while police and firefighters swept the school for suspicious packages.
They found nothing.
The school took two calls, he said. The first was garbled so the call taker hung up. The second was a message from a male voice that sounded to be recorded.
Once the all-clear was given, pupils at the school went to an assembly and were sent home about 2.30pm.
Rosebank School office manager Gail McElray said that the school had not received a bomb threat and was not evacuated, though access to the school was blocked after police cordoned off Frances St.
Police would now try to figure out the "origin and meaning" behind the phone call, Sgt Bull said.
Senior Sergeant Al Dickie, area response manager for the Clutha Taieri said emergency services continued to be frustrated when hoax calls were made as it consumed valuable personnel that might need to respond to other priority incidents.
- Sam White and Vaughan Elder
