Moffats Boys' Preparatory School spent six years at Lifton, and the organiser of the plaque, Colin Shaw-Stewart, said those years were 'some of the most informative and enjoyable times in my life'.
'I have always felt there should be some permanent record there,' he said.
The ceremony will be attended by contemporaries of Mr Shaw-Stewart, some of whom are travelling from Scotland for the event.
They will be joined by a number of present boys, girls and staff and headmaster Mark Daborn, who will be holding their annual summer camp in Cornwall.
The service has been arranged by the Rev John Heath, and the plaque — which has been engraved on Delabole Slate — will be unveiled by Esme Engleheart, daughter of the Rev H T Boultree, who was rector of Lifton from 1948 to 1954.
Mrs Engleheart's husband, David, was a former joint head of Moffats with his late brother, John.
John ran the school at Lifton Park throughout the war with his mother, Natalie, who founded the school in 1934 and was the writer of children's books and poems.
The school was one of many evacuated to large West Country homes, and few could have foreseen that a whole school generation would pass before they could return home.
Moffats later outgrew its home north of London and moved to Knight Hall in Shropshire.
The service at Lifton Church is at 2.30pm.