The grenade exploded soon after the girl lifted the unknown object from the ground out of curiosity, Superintendent of Police Baburam said.
While the 12-year-old girl was killed on the spot, four other girls were injured.
The injured were admitted to a local hospital from where the one of the seriously injured girl was referred to the Anugrah Narayan Medical College and Hospital (ANMCH) here.
The explosive might have been left over by the naxalites, Baburam said.