DAMATURU, Nigeria - At least 20 people, most of them students, were killed in a bomb blast at a secondary school in Nigeria's northeastern town of Potiskum on Monday as they gathered for assembly before classes, a teacher said.No one immediately claimed responsibility for the attack in Yobe State, a territory that has been hit by an insurgency by Islamist Boko Haram rebels. At least 29 people were killed in another bombing in the town on Nov. 3."As it stands now, we have taken about 20 dead bodies to the hospital. There are some that are critically injured and I am sure the death toll will rise," the teacher at the school told Reuters, asking to remain anonymous.