Large learning gains in pockets of extreme poverty: Experimental evidence from Guinea Bissau

•In some extremely poor remote regions most children grow up illiterate and innumerate.•A cluster-RCT in rural Guinea Bissau evaluated a concerted, supply-based intervention.•The intervention consisted of running schools in lieu of the government.•The intervention dramatically raised learning levels relative to controls.•Such concerted intervention may be necessary to achieve transformative learning gains.