Implicitly, this simulation results in a cost saving of 50 percent (removing half the teachers). Our estimates suggest that learning outcomes could be maintained if a doubling of class sizes reduces learning by no more than 0.04 SD. A similar exercise suggests that even for the highest class-size effect considered, a cost saving of 12 percent could be achieved (by transferring the students of the 12 percent of teachers with the lowest content knowledge) without compromising learning.

_Long-run effects of increasing teacher knowledge and time spent teaching_ The last policy simulation we consider is the joint effect of increasing both teacher knowledge and effort. The provision of primary education has expanded greatly in low- and middle-income countries in the last two decades, including countries in Sub Saharan Africa. Among current teachers in the SDI data, we find that twice as many teachers have entered the profession in the last 10 years than in the decade before. This expansion of the teaching force will likely continue: According to recent population projections, close to half the world’s children will live in Africa by the end of the 21st century (UNICEF, 2014). The number of primary school age children in Sub-Saharan Africa is set to rise from about 170 million to 206 million between 2019 and 2029, reaching 270 million by 2050.<sup>50</sup> As illustrated in Figure 6, simply to keep pace with population growth—adjusting for teacher retirement—and to maintain pupil-teacher ratios at a rough benchmark of 40 students per teacher, would require the hiring of 1.6 million new teachers by 2030 and close to 5 million by 2050. As the large majority of teachers are employed on permanent and pensionable civil service contracts, and as upgrading current teachers’ knowledge is probably significantly more costly than raising the quality of new cohorts of teachers, an expansion of the teaching force at low knowledge represents a lost potential with durable impacts.<sup>51</sup> We use the estimate of the cumulative impact of content knowledge to simulate the impact of a reform that raises teachers’ knowledge over time to the minimum entry requirement for teacher training; i.e., lower secondary education (10 effective years of education). We treat the knowledge of current teachers as fixed, which means that average teacher content knowledge can only be raised by hiring new qualified teachers, and we assume that teachers work for 40 years. We assume teachers follow their class up to grade six

> teachers in response to findings from an evaluation showing that two-thirds of the lower primary teachers failed to score 75% or higher on assessments/exams set for their students.  Perhaps unsurprisingly, the proposal led to substantial controversy and political push-back.

> 50 See appendix for details for these projections and the policy simulations.

> 51 Note though that in countries where contract teachers are prevalent, and for teachers with less than 10 years of experience, almost half of the teachers are employed on short-term contracts. This partly reflects an age and a cohort effect, as many contract teachers graduate to civil service status over time (Bold et al., 2017).

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