The UN says about eight million children in 10 of Nigeria's states do not go to school.
The UN children's fund said the average school enrollment rate in these areas was just 50%.
The majority of those out of school are girls, and the states most affected are in northern Nigeria.
Unicef said more than 10 million children were unable to access safe education, largely due to an Islamist insurgency in the north-east of the country.
Hundreds of girls and boys have been abducted from their schools by Boko Haram jihadists.