He joined the Army Service Corps soon after the outbreak of war (Service No. T4/1856030) and he appears on the Roll of Honour published in the Shaftesbury Grammar School Magazine in October 1914. By March 1915 he had been promoted to the rank of Sergeant and the June edition of the magazine reported: “W.Jeffery visited the school on March 31st. Owing to an attack of mumps he was unable to sail with his regiment to the Dardanelles, and is now applying for a commission with the A.S.C.”
William received a commission and served in France as a Second Lieutenant in the 5th Rifle Brigade. The Grammar School Magazine reported: “William Jeffery was wounded in the eye and sent over to a hospital at Chelsea. For a time he made splendid progress, but had a relapse and died suddenly.” He died on 7th October 1917. The death was recorded as a civilian death in the Chelsea Registry, but he was buried in the military section of Andover Cemetery. He is remembered on the Andover Cenotaph and on the Shaftesbury Grammar School Memorial, now in Shaftesbury School.
A "First List of Old Shastonians Serving in H.M. Forces" was produced by Shaftesbury Grammar School in September 1915. It shows the regiment in which William served and the date and term in which he left school. Throughout the war a hand-written list was also compiled of Shaftesbury Grammar School old boys serving in the armed forces. An asterix can be seen next to William's name, indicating that he had died. The board is now at Gold Hill Museum.
First List of Old Shastonians Serving in H.M. Forces, September 1915
Shaftesbury Grammar School Magazine, October 1914
Shaftesbury Grammar School Magazine, March 1915
Shaftesbury Grammar School Magazine, June 1915
Shaftesbury Grammar School Magazine, November 1917