A school in central Nigeria collapsed on Friday killing at least 21 people, mostly pupils as they were sitting their exams, the Red Cross and witnesses said. Trapped pupils cried for help under the rubble as desperate parents looked for their children after the Saint Academy school in Jos North district of Plateau State fell in on classrooms, according to an AFP correspondent at the scene. Rescue workers tried to reach the victims with heavy machinery and images from the scene showed crowds gathering around a caved-in concrete building and heaps of debris. Red Cross spokesman Nuruddeen Hussain…