Children from poorer communities as well as girls, the disabled, immigrants and ethnic minorities were at a distinct educational disadvantage in many countries, the UN's Paris-based education body UNESCO said.
The report noted that in low- and middle-income countries, adolescents from the richest 20% of households were three times more likely to complete the first portion of secondary school - up to age 15 - than those from poor homes.
In 20 poor countries, mainly in sub-Saharan Africa, hardly any rural girls complete secondary school, UNESCO said.
Two African countries still ban pregnant girls from school, 117 countries allow child marriages, and 20 have yet to ratify an international convention that bans child labour.
UNESCO urged countries to focus on disadvantaged children when schools reopen after coronavirus lockdowns.
