By Imogen Foulkes
BBC correspondent in Geneva
The United Nations' target of universal primary education by 2015 is unlikely to be reached, a new report shows.
Many teachers' salaries are too low to meet basic living standards
More children are going to school, but many are dropping out before finishing primary education, the UN educational, scientific and cultural agency said.
Many more are leaving without having learned even basic literacy and numeracy skills, the UN said.
Unesco's 2005 Education For All report also reveals a continuing gender inequality in education.
Two thirds of children who do not attend school are girls. Two thirds of the 800 million illiterate people around the world are women.
The report monitors the progress of 160 countries towards achieving universal education.
School attendance is increasing, but not fast enough, the report found.
Around the world, over 100 million children do not go to school.
Moreover, an improvement in school attendance has not, in mainly developing countries, been matched by an increase in teachers or in education funding, the report said.
Unesco's Ingeborg Breines said that getting children into the classroom is only part of the task.
"For many years the developing countries have thought that the main bottleneck was to get the children to school."
"Little by little they have experienced that if the quality is not satisfactory and the content of what is being taught is not relevant to the learners, they will drop out," she added.
In some African countries Unesco found teachers struggling with classes of 60 or more children.
Teacher's salaries were often not enough to guarantee basic living standards and teacher training was rudimentary.
In some schools mathematics teachers had only basic numeracy skills. They often scored less than their own pupils on tests.
Such lack of quality leads to high dropout rates, and to children leaving school having learned very little.
Unesco wants greater political commitment to schooling, starting with a promise by governments to allocate at least 4% of their Gross Domestic Product (GDP) to education.