Aisha Community School has 584 pupils, and 60 desks. “Sometimes we lay planks across them so the children have something to write on,” the headmaster tells us. Even by Zambian standards, this is not a wealthy school. Aisha occupies a tiny maze of concrete rooms in the middle of Ng’ombe, a high-density slum area in the suburbs of Lusaka, surrounded by box-like dwellings and brown dirt roads. The h…