Free primary education was introduced in January 2003, attracting students as old as 20 to education for the first time, sharing classes with children 15 years their junior.
Ayany Estate School just outside Kibera was set up by UNICEF in 1990, but now receives government funding for text books and writing materials.
'But this only allows for one pencil per year per pupil so parents have to contribute, ' says one teacher.
The government target is for 50 pupils per class, but the 2,000 pupils of Ayany are squashed into classes of between 80 and 100 children.
A nutrition programme has also been introduced and UNICEF sends bags of maize and beans every month.
As there are no cooking facilities parents of the pupils are now clubbing together to help build a new kitchen.