On the surface, the schools appear only as playgrounds or soccer fields, with classrooms sunk like bunkers under yards of dirt. Children get a glimpse of the sky only occasionally, when all-clears from air raid alerts allow a bit of time outside.

During the first day of classes on Tuesday in the southern Ukrainian city of Zaporizhzhia, students spent their recess hanging out under ventilation ducts or sitting on benches along the reinforced concrete walls. Little feet swung above the floor.

There are now 74 of these wholly underground schools in Ukraine, a stark measure of the drawn-out nature of the war and of a campaign of Russian aerial bombardment that is only escalating more than four years in.

“It’s impossible to get an education now without going underground,” said Oleksandr Lagin, 39, a grocery store manager whose two sons study below ground for safety against drones and missiles.

It might seem apocalyptic to put children in such schools. But in some ways they are the lucky ones.

Most of Ukraine’s 3.6 million school-age children study online or above ground. During air alerts, they descend to basements, where students of all ages crowd together and instruction is impossible.

In Kyiv, the capital, on Friday, for example, air alerts were in effect for 158 minutes, or the equivalent of four of six lessons in a school day. The city has been under near-constant assault from jet-powered drones for days, unnerving parents. On Tuesday, an air alert disrupted opening-day ceremonies there.

The authorities in the Kyiv region urged schools to avoid any ceremonies involving crowds, lest a drone hit them. “Put safety first,” implored Tymur Tkachenko, the regional military administrator.

As Russia continued its barrages, eight people were killed overnight on Tuesday in Kyiv, including six employees of a railway company in an attack on train infrastructure.

Zaporizhzhia, a steel industry town less than 10 miles from the front line, has for years been a shooting gallery for Russian bombs and drones. The city has built 10 underground schools with European funding, and there are 22 in the wider region.

Paradoxically, these frontline schools in Zaporizhzhia — generally a far more dangerous place than Kyiv — are better prepared to keep children in class.

Yulia Barabash arrived at the playground of the underground Sichovy Collegium with her lanky 6-foot-tall son, Damir, 13, who was starting eighth grade. Then they descended a flight of stairs to reach the classrooms.

Children scurried about the blue-painted concrete tunnels, wearing traditional embroidered shirts and carrying flowers to give to their teachers on the first day during a ceremony called the “first bell.”

Ms. Barabash said her son had spent the summer playing video games and taking taekwondo classes. She was glad he was back in school, she said, not least because he is far safer there than above ground at home.

“I like studying here,” Damir said. “The teachers are really kind, and it feels safe.”

He also studied underground last year. Many days, he said, he is underground all day. “Sometimes when we have a break between lessons, they allow us to get out for fresh air,” he said. “But if there is an alarm, you have to come back.”

He likes to play soccer during physical education class but is disappointed when games are interrupted by air alerts.

Ms. Barabash gushed with enthusiasm for underground schooling. “I am really, really thankful,” she said. “I bring him here in the morning and don’t worry all day.”

For the children, a reminder of the war aboveground appears in a hallway as a placard honoring the four former students and one teacher who have died as soldiers in the army. Fresh sunflowers in vases adorned the memorial. “Eternal Memory to Our Heroes,” it said.

Dmytro Sizhykh, the father of Danylo, an eighth grader at the school, is fighting in the military along the mostly stalled but still lethally dangerous front line. He also is a proponent of underground schooling.

“I can do my job without worrying about my son,” he said by telephone. “If there were no underground schools and children had to study in regular ones, I wouldn’t let him go.”

Some students mixed pride in their school with some grumbling about the subterranean setting. “We like the school but cannot go outside because of the war,” said Maria Guchkova, 14, an eighth-grade student who likes to study English.

The war has taken a toll on learning, as it has with many other aspects of Ukrainian life. Scores on standardized tests have fallen during the war, said Tetiana Zheriobkina, an education analyst at Cedos, a research organization in Kyiv.

Studies show a correlation between war-related childhood anxiety and falling scores, she said. “Year after year, indicators are not improving,” she said.

The country has 11,940 schools, of which about 2,000 are online only. Most mix online and in-person education. Some, through online classes, secretly educate Ukrainians in areas occupied by Russia.

By law, schools everywhere in Ukraine must have a readily accessible bomb shelter to carry out any in-person instruction. These are typically just basements, safe but unusable for instruction.

The authorities in the capital are considering building classrooms in basements for younger grades. Principals will make the final decisions on where instruction can be continued during air alerts.

Elsewhere in Ukraine, cities have built underground schools or repurposed subterranean spaces in a range of sites like movie theaters, hospitals, day-care centers, water pumping stations and natural-gas infrastructure.

In Zaporizhzhia, the Sichovy Collegium has bunker amenities that tin-pot dictators or doomsday preppers might envy, including a top-of-the-line ventilation system capable of filtering radiation. It is linked to a backup diesel generator.

Halyna Volkova, the principal, said test scores were only one measure of education in Ukraine today. She said she had high hopes for Ukraine’s war generation, despite being raised in a crucible of danger, loss and fear.

During an interview in the hallway, she turned and said, “Hello, my sunshine” and gave a quick hug to a passing boy she recognized from last year.

Her school, she said, tries to teach pride and patriotism along with academics, qualities she said would be needed in these children’s futures. “Overall development is more important,” she said.

Besides, she said, a year ago, she had a student who nailed the nationwide standardized graduation test with a perfect score.

Above all, she said, a good education is a safe education.

In an escalating drone war, that means going underground. Ms. Volkova was upbeat about the future of underground education.

“We don’t say everything is terrible and apocalyptic,” she said, “because this is our life.”