The walls are crumbling, the windows shattered, and the boys sit three to a desk. But by being enrolled in classes at all, the pupils are among the luckiest children in war-torn Yemen.

In the rebel-held capital Sanaa, students in olive green uniforms lined up for a morning salute at the Al-Wahda boys' school. "Onwards!" nearly 70 pupils chanted in unison, reaching forward to form a human chain. But 15-year-old Alaa Yasser was not among them. Instead, he was working at a nearby car shop to support his family. "I had to stop going to school to work with my father to help him earn a living," said Yasser, whose family fled the southwestern city of Taez.

Two million children across the country have no access to education, according to the U.N. children's agency (UNICEF), three years into a war that has pushed Yemen to the brink of famine and shows no sign of waning.

Yemen's war prompted the already weak economy to collapse and, coupled with a blockade of its ports and airport, people are struggling to survive as prices skyrocket. More than 22 million people, three-quarters of the population are in need of humanitarian assistance, according to the United Nations.

Yemeni children face significant risks, with a high proportion of girls marrying at an early age. More than 40 percent are married before the age of 15, while three-quarters wed by 18, according to UNICEF.

UNICEF estimates 4.5 million children risk losing access to state schools in Yemen, as teachers have not been paid in nearly two years. More than 2,500 schools have been damaged or destroyed, while some are now used as shelters for displaced people or as camps run by armed groups.Yemen has been wracked by conflict since 2014, when Shiite Houthi rebels overran much of the country. Saudi Crown Prince Mohammed bin Salman, the former defense minister, and Saudi Arabia's allies launched Operation Decisive Storm in March 2015. Since then, the bombings continue to haunt pupils.