The 1962 artwork was inspired by Dame Barbara’s 1937 carving in lignum vitae of the same name, which is displayed at the Courtauld Gallery in London.
The sculptor had donated it to a Save the Children charity auction in 1963, where it was bought by the former governor who later bequeathed it to Bryanston School.
The school said the Hepworth Estate had lost track of the piece until it was recognised by its newly appointed archivist, Laura Sweetenham.
Guy Schwinge of the art advisers Hanover Forbes described it as “a remarkable work by the most important female sculptor of the 20th century”.
Headmaster Richard Jones said: “The rediscovery of this statue means we have the exciting prospect of providing opportunities for talented students who might not otherwise be able to benefit from a Bryanston education.”
Single Form will be auctioned by Duke’s, Dorchester, on 17 April.
