We conducted a survey covering 20% of villages with 200-1000 population in rural Guinea-Bissau. We interviewed household heads, care-givers of children, and their teachers and schools. We analysed results from 9,947 children, aged 7-17, tested for literacy and numeracy competency. Only 27% of children were able to add two single digits, and just 19% were able to read and comprehend a simple word. Our unannounced school checks found 72% of enrolled children in grades 1-4 attending their schools, but the schools were poorly equipped. Teachers were present at 86% of schools visited. Despite surveying 351 schools, we found no examples of successful schools where children reached reasonable levels of literacy and numeracy for age. Our evidence suggests that interventions that raise school quality in these villages, rather than those which target enrolment, may be most important to generate very sharp improvements in children's educational outcomes. [Additional financial support for this research was provided by Effective Intervention, a UK charity.]