The benefits of exclusive breastfeeding (EBF) for infant health and survival are well documented. However, its impact on educational outcomes has been contested and poorly researched in Africa. It has been hypothesised that positive associations reported in high-income countries can be attributed to residual confounding by socioeconomic status (SES). Our study investigated whether EBF duration in infancy is associated with educational attainment and age-for-grade attainment trajectories at school-age in rural Malawi. Longitudinal data on 1021 children at the Karonga demographic surveillance site in Malawi were analysed. Breastfeeding data were collected 3 months after birth and again at age one. The school grade of each child was recorded each year from age 6 until age 13. We calculated age-for-grade based on whether a child was at, over, or under the official expected age for a grade. Generalised estimating equations estimated the average effect of breastfeeding on age-for-grade. Latent class growth analysis identified age-for-grade trajectories, and multinomial logistic regression examined their associations with EBF. Maternal-child characteristics, SES, and HIV status were controlled. Overall, 35.9% of the children were exclusively breastfed for 6 months. Over-age for grade steadily increased from 9.6% at age 8 to 41.9% at age 13. There was some evidence that EBF for 6 months was associated with lower odds of being over-age for grade than EBF for less than 3 months (aOR = 0.82, 95%CI = 0.64-1.06). In subgroup analyses, children exclusively breastfed for 6 months in infancy were less likely to be over-age for grades between ages 6-9 (aOR = 0.64, 95%CI = 0.43-0.94). Latent class growth analysis also provided some evidence that EBF reduced the odds of falling behind in the early school grades (aOR = 0.66, 95%CI = 0.41-1.08) but not later. Our study adds to the growing evidence that EBF for 6 months has benefits beyond infant health and survival, supporting the WHO's recommendation on EBF.