Dr. Mohammad Dawood Danish, the head of the Mohammad Ali Jinnah hospital in Kabul, said that 20 bodies and more than 40 wounded people were transferred to his hospital. Most of them were students, he said.

“The health condition of a number of girls is critical,” Dr. Danish said. Tariq Arian, a spokesman for the Interior Ministry, said 52 people had been wounded in all.

The presidential palace in Afghanistan blamed the killings on the Taliban, calling them “a crime against humanity,” in a statement.

What happened Saturday, for all its brutality, has become painfully common for Kabul, a capital city that has been rocked by terrifying violence — suicide vests, rocket barrages, massive truck bombs — for years.

But the attack on Sayed Ul-Shuhada high school comes at an inflection point, as U.S. and international forces leave and the next chapter of Afghanistan’s enduring war begins to unfold. Many residents saw the massacre, which left books and backpacks and bodies scattered across the ground on what had been a pleasant spring afternoon, as a foreshadowing of what is to come.

“I have lost count of attacks harming children. I have lost count of attacks on education,” Shaharzad Akbar, the chairwoman of the Afghanistan Independent Human Rights Commission, said on social media. “I have lost count of civilians killed even just this month. This war must stop. This madness, this hurt, this pain.”

Mohammad Hussain Jawhari, a resident of the area, said three rockets were fired at the gates while the girls were leaving the school. Another witness said the blasts were caused by multiple car bombs. Mr. Arian, the Interior Ministry spokesman, said the nature of the explosion was unclear.