Millions of American schoolchildren are receiving free or low-cost meals for the first time as their parents, many once solidly middle class, have lost jobs or homes during the economic crisis, qualifying their families for the decades-old safety-net program.

The number of students receiving subsidized lunches rose to 21 million last school year from 18 million in 2006-7, a 17 percent increase, according to an analysis by The New York Times of data from the Department of Agriculture, which administers the meals program. Eleven states, including Florida, Nevada, New Jersey and Tennessee, had four-year increases of 25 percent or more, huge shifts in a vast program long characterized by incremental growth.

The Agriculture Department has not yet released data for September and October.

“These are very large increases and a direct reflection of the hardships American families are facing,” said Benjamin Senauer, a University of Minnesota economist who studies the meals program, adding that the surge had happened so quickly “that people like myself who do research are struggling to keep up with it.”

In Sylva, N.C., layoffs at lumber and paper mills have driven hundreds of new students into the free lunch program. In Las Vegas, where the collapse of the construction industry has caused hardship, 15,000 additional students joined the subsidized lunch program this fall. Around Rochester, unemployed engineers and technicians have signed up their children after the downsizing of Kodak and other companies forced them from their jobs. Many of these formerly middle-income parents have pleaded with school officials to keep their enrollment a secret.

Students in families with incomes up to 130 percent of the poverty level — or $29,055 for a family of four — are eligible for free school meals. Children in a four-member household with income up to $41,348 qualify for a subsidized lunch priced at 40 cents.

Among the first to call attention to the increases were Department of Education officials, who use subsidized lunch rates as a poverty indicator in federal testing. This month, in releasing results of the National Assessment of Educational Progress, they noted that the proportion of the nation’s fourth graders enrolled in the lunch program had climbed to 52 percent from 49 percent in 2009, crossing a symbolic watershed.

In the Rockdale County Schools in Conyers, Ga., east of Atlanta, the percentage of students receiving subsidized lunches increased to 63 percent this year from 46 percent in 2006.

Steve Hebert for The New York Times

More than 100 students eat a free dinner daily after classes at Ingels Elementary School in Kansas City, Mo. The Hickman Mills C-1 district feared students would otherwise go to bed hungry.

“We’re seeing people who were never eligible before, never had a need,” said Peggy Lawrence, director of school nutrition.

One of those is Sheila Dawson, a Wal-Mart saleswoman whose husband lost his job as the manager of a Waffle House last year, reducing their income by $45,000. “We’re doing whatever we can to save money,” said Ms. Dawson, who has a 15-year-old daughter. “We buy clothes at the thrift store, we see fewer movies and this year my daughter qualifies for reduced-price lunch.”

She added, “I feel like: ‘Hey, we were paying taxes all these years. This is what they were for.’ ”

Although the troubled economy is the main factor in the increases, experts said, some growth at the margins has resulted from a new way of qualifying students for the subsidized meals, known as direct certification. In 2004, Congress required the nation’s 17,000 school districts to match student enrollment lists against records of local food-stamp agencies, directly enrolling those who receive food stamps for the meals program. The number of districts doing so has been rising — as have the number of school-age children in families eligible for food stamps, to 14 million in 2010-11 from 12 million in 2009-10.

“The concern of those of us involved in the direct certification effort is how to help all these districts deal with the exploding caseload of kids eligible for the meals,” said Kevin Conway, a project director at Mathematica Policy Research, a co-author of an October report to Congress on direct certification.

Congress passed the National School Lunch Act in 1946 to support commodity prices after World War II by reducing farm surpluses while providing food to schoolchildren. By 1970, the program was providing 22 million lunches on an average day, about a fifth of them subsidized. Since then, the subsidized portion has grown while paid lunches have declined, but not since 1972 have so many additional children become eligible for free lunches as in fiscal year 2010, 1.3 million. Today it is a $10.8 billion program providing 32 million lunches, 21 million of which are free or at reduced price.

All 50 states have shown increases, according to Agriculture Department data. In Florida, which has 2.6 million public school students, an additional 265,000 students have become eligible for subsidies since 2007, with increases in virtually every district.

“Growth has been across the board,” said Mark Eggers, the Florida Department of Education official who oversees the lunch program.

In Tennessee, the number of students receiving subsidized meals has grown 37 percent since 2007.

“When a factory closes, our school districts see a big increase,” said Sarah White, the state director of school nutrition.

In Las Vegas, with 13.6 percent unemployment, the enrollment of thousands of new students in the subsidized lunch program forced the Clark County district to add an extra shift at the football field-size central kitchen, said Virginia Beck, an assistant director at the school food service.

In Roseville, Minn., an inner-ring St. Paul suburb, the proportion of subsidized lunch students rose to 44 percent this fall from 29 percent in 2006-7, according to Dr. Senauer, the economist. “There’s a lot of hurt in the suburbs,” he said. “It’s the new face of poverty.”

In New York, the Gates Chili school district west of Rochester has lost 700 students since 2007-8, as many families have fled the area after mass layoffs. But over those same four years, the subsidized lunch program has added 125 mouths, many of them belonging to the children of Kodak and Xerox managers and technicians who once assumed they had a lifetime job, said Debbi Beauvais, district supervisor of the meals program.

“Parents signing up children say, ‘I never thought a program like this would apply to me and my kids,’ ” Ms. Beauvais said.

Many large urban school districts have for years been dominated by students poor enough to qualify for subsidized lunches. In Dallas, Newark and Chicago, for instance, about 85 percent of students are eligible, and most schools also offer free breakfasts. Now, some places have added free supper programs, fearing that needy students otherwise will go to bed hungry.

One is the Hickman Mills C-1 district in a threadbare Kansas City, Mo., neighborhood where a Home Depot, a shopping mall and a string of grocery stores have closed.

Ten years ago, 48 percent of its students qualified for subsidized lunches. By 2007, that proportion had increased to 73 percent, said Leah Schmidt, the district’s nutrition director. Last year, when it hit 80 percent, the district started feeding 700 students a third meal, paid for by the state, each afternoon when classes end.

“This is the neediest period I’ve seen in my 20-year career,” Ms. Schmidt said.