A car bomb went off outside the Sayed-ul-Shuhada school in a Shiite-majority neighbourhood in western Kabul, the Tolo News agency said. It was followed by two more blasts.
The majority of victims are young girls.
Afghan President Ashraf Ghani has declared Tuesday a national day of mourning. He blamed the blasts on the Taliban, but a spokesman for the militants denied they were involved and accused the Daesh* terror group of attacking the school.
*Daesh (IS/ISIL/ISIS) is a terrorist organisation banned in Russia.