The device was found at Corfe Mullen, Dorset on Tuesday morning.
A Military Explosive Ordnance Disposal team (EOD) has set up a 1,000m (0.62 mile) exclusion zone at Henbury Quarry in Old Market Road, ahead of a controlled explosion at 10:00 GMT.
Local roads within the cordon and the A350 will be closed from 09:30.
Superintendent Caroline Naughton, of Dorset Police, said: "I would like to reassure residents that there is no immediate risk to anyone in the local area.
"When the controlled explosion takes place it is anticipated that a loud bang will be heard in the area."
A rest centre has been set up for residents at the Memorial Hall, Sturminster Marshall.
People are being urged to take any pets or medication with them.
Dorset Police said staff and pupils from Castle Court School are being taken to another site.
The cordon is expected to be lifted by 11:00 GMT.
Homes, businesses and a school are being evacuated in a village after a World War Two bomb was found in a quarry.