The poorest countries are typically the ones with the lowest education rates. Some countries like this are Liberia, Malawi, Central African Republic, etc.
Towards the end of 2003, there were roughly 113 million children worldwide that were not enrolled in school.
The dropout rates in these countries are quite extreme for multiple reasons, most relating to having to work to earn money for whatever reason. For example, ever since the year 2000, the primary school completion rate in Bangladesh was only 60%.