The pupil was among a group who were creating some shore art during their two-night stay in the island.
Bomb disposal officer Simon Hamon said that the boy immediately told his teachers that he thought he had found an unexploded bomb, and they in turn contacted Lihou Charitable Trust warden Richard Curtis, who alerted the bomb disposal team.
On arriving, PC Hamon and his colleague PC Stuart Allan discovered that the shell was a 75mm high explosive round, which he said could have been fired by a tank or a piece of field artillery, probably during an exercise carried out during the Occupation.
Within about 5m of the shell the officers found its fuse cone, and the two elements were put together before a controlled explosion was carried out.
