At least 31 persons, mostly children, were killed when a school bus plunged into a gorge in Tanzania on Saturday, officials said.Theresia Mahongo, Karatu District Commissioner, said the bus plunged into the Marera gorge killing 28 students and three teachers near Arusha, Xinhua news agency reported.According to Mahongo, the school bus had 35 people on board.The students were supposed to participate in inter-school mock examinations in the district.