It was the noon hour, and children were playing outside the school, squeezing in their last few minutes of fun before lessons began. Suddenly, there came the roar of helicopters overhead.

Bhone Tayza, 7, looked up. His cousin shouted at him to run, and both of them dashed to hide in a hole in the trunk of a tamarind tree. Then Bhone Tayza remembered he had left his school bag in his classroom and ran back to get it. Soldiers started firing rockets.

When his mother heard the school had been attacked, she said she rushed to the scene, her account of her son’s final moments largely corroborated by a teacher there. She begged soldiers to let her in. “Mommy,” she heard a familiar voice cry. A soldier allowed her into the building, where she saw her only son in a pool of blood.

“I just want to die,” she said he told her, in a faint voice. “I can’t stand the pain.”

He died soon afterward, as did 10 of his fellow students.

More than 13,000 children have been killed in the bloodshed that began when Myanmar’s army seized power early last year. But the Sept. 16 strike on the school, in Let Yet Kone village in central Myanmar, killed more of them than any single episode since the coup. The United Nations secretary general, António Guterres, condemned the attack.

For more than a year, the army has been battling resistance fighters, many of them ordinary people who have taken up arms and formed groups called the People’s Defense Forces. Each day brings news of more people dying, mostly civilians.

But the images that surfaced from the attack on the school — dead children wrapped in cloth, an abandoned school bag next to blood spatters, small sandals sprayed with rubble — still had the capacity to shock.

For many in Myanmar, the attack hardened their resentment against the military and renewed their anguish at the world’s failure to intervene.

“I want to ask the international community: How many children must be killed in our country before Min Aung Hlaing can be overthrown?” said Bhone Tayza’s mother, referring to Myanmar’s commander in chief. She declined to be named for fear of retribution.

The United Nations special rapporteur on human rights in Myanmar, Tom Andrews, told the U.N. Human Rights Council in Geneva this week that conditions had “gone from bad to worse, to horrific for untold numbers of innocent people in Myanmar.”

In a telephone interview on Friday, Mr. Andrews said the attack on the school was “another unspeakable horror that has been systematically inflicted on the people of Myanmar.”

“This is a war crime,” he said.

On Tuesday, the junta’s spokesman, Zaw Min Tun, said Let Yet Kone village had been harboring members of the People’s Defense Forces and their allies from the Kachin Independence Army, an ethnic rebel group. They had used the villagers as human shields, he said at a press briefing.

He played a video of two teachers from the school, who had been arrested after the strike. They said members of the People’s Defense Forces had attacked soldiers, then rushed to the school to take cover, forcing the soldiers to fire on it to defend themselves.

Mr. Zaw Min Tun said the army had “even saved two children’s lives” by taking them to a hospital by helicopter. He denounced “the cunning media for publishing that we shoot children.”

Villagers disputed his account. They said that no one from the People’s Defense Forces had been in the school, and that the two teachers had made their statements under duress.

One villager said he had seen four Russian-made helicopters — two Mi-35Ms and two Mil Mi-17s — carry out the attack, firing rockets and dropping soldiers onto the school grounds.

Let Yet Kone village is in the Sagaing region, a stronghold of the resistance. For months, the army has been trying to regain control of the area.

The roads are controlled by the People’s Defense Forces, so the military relies heavily on air bombardment. Military commanders have fled their offices in the region. Almost daily, there are battles between the army and the rebels, as well as bombings by urban guerrillas.

The school, located in a monastery, was set up in secret after the coup. Teachers in the village, like thousands of others throughout Myanmar, have been on strike since the coup, refusing to work in government schools as part of a nationwide protest movement.

But many went on to teach in schools that were set up privately, like the one in Let Yet Kone, or established by the National Unity Government, a shadow government in exile. The junta has outlawed such schools, arresting teachers as well as support staff, like drivers who deliver textbooks.

The volunteer teachers at the Let Yet Kone school, where 249 elementary and middle school students took lessons, had taught the children how to hide in the event of an airstrike. That, said one teacher, is why most of them escaped — except for some of the youngest, who struggled to remember what to do.

The teacher, who also declined to be identified for fear of retribution, said a rocket landed near where she was hiding with several children. One child was killed, she said, the force of the impact so strong that his scalp was blown off and stuck to a wall.

Soldiers entered the school, demanding that everyone come out of hiding. According to the teacher, they told the frightened staff and students to say later that resistance fighters, not the army, had started the battle. The teacher said she saw children without limbs, many of them drenched in blood.

Su Yati Hlaing, 7, did not make it out alive. Her parents had been working in Thailand for five years to make more money for the girl and her 10-year-old sister.

“But now our family can never be complete,” said Su Yati Hlaing’s father, who declined to be named, fearing government persecution. “I will never forgive the army.”

After the attack, soldiers took the bodies to another town and cremated them, according to villagers. The bereaved parents have not gotten their children’s ashes back.

A week after the assault, most of the 3,000 villagers in Let Yet Kone have evacuated to other neighboring villages, like more than a million others who have lost their homes in the conflict.

Many of the villagers now live under a pall of grief and fear, still trying to come to terms with the trauma of the attack.

“I still cry when noon comes,” the teacher said.