At least 52 people have been injured in the blast that took place in western Kabul, and according to Interior Ministry spokesman Tariq Arian, most of the injured are students at the school.
An investigation into the "terrorist attack" has been launched, according to the ministry.
Ghulam Dastagir Nazari, a spokesman for the Afghan Ministry of Health, said 46 people were currently receiving hospital care.
The mood in the Afghan capital has been tense since Washington last month announced that all US troops in the country will be withdrawn on September 11 this year. Following that announcement, the Taliban movement has increased the number of attacks across the country, according to the Afghan authorities.