MOGADISHU, Somalia—A large explosion outside a school in Somalia’s capital on Thursday killed at least eight people, including students, witnesses said. The extremist group al-Shabab claimed responsibility for the attack. The al-Qaida-linked group controls large parts of rural Somalia and continues to frustrate efforts at rebuilding the Horn of Africa nation after three decades of conflict. The blast sent a plume of smoke above a busy part of Mogadishu during the morning rush hour. The blast shredded part of the school, with emergency workers looking through the collapsed roof beams and wooden benches. “We were extremely terrified by the blast,” said teacher Mohamed Osman, who said he was explaining a lesson to students when the explosion occurred. “We were disorientated and we were covered with dust and smoke.” Police spokesman Abdifatah Adam Hassan said eight people were killed and 17 others wounded. “This is a tragedy,” said Abdulkadir Adan of …