NEW ORLEANS — The storm that swamped this city three years ago also effectively swept away a public school system with a dismal record and faint prospects of getting better. Before Hurricane Katrina, educator John Alford said, he toured schools and found “kids just watching movies” in classes where “low expectations were the norm.”

Now Mr. Alford is one of many new principals leading an unparalleled education experiment, with possible lessons for troubled urban schools nationwide. New Orleans, in a post-Katrina flash, has become the first major city in which more than half of all public school students attend charter schools.

For these new schools with taxpayer funding and independent management, old rules and habits are out. No more standard hours, seniority, union contracts, shared curriculum, or common textbooks. In are a crowd of newcomers — critics call them opportunists — seeking to lift standards and achievement. They compete for space, steal each other’s top teachers, and wonder how it is all going to work.

Mr. Alford, 33, launched Langston Hughes Academy for kindergarten through sixth grade in a stately, yellow, peachy-red Mid-City school building that withstood eight feet of floodwater after the August 2005 hurricane. One day this spring, he strolled a third-floor corridor that had fresh paint, student work and college banners on the walls. “Thinking about my kids keeps me up at night,” he said, “but the larger mission is there — like, wow … we will never have this chance again, and if it is successful, other cities should do it, too.”

Some cities are moving in this direction, but none has ever moved so far, so fast. Three in every 10 D.C. public school students are in charters, a much larger percentage than in most cities. The New Orleans charter school penetration rate is much greater: 53% of the post-Katrina enrollment of 33,200 students, according to school officials. Before the hurricane, charters had about 2% of the city’s 67,000 public students.

Many parents in the years since the storm have sought spaces in Roman Catholic and other private schools, but charters have become the most popular option because they are free. Charter leaders acknowledge that their schools must produce achievement gains or the experiment will flop.

Nationally, research shows little difference between average test scores for charters and for regular public schools. Experts say the quality of charter schools varies as much as the quality of regular schools.

Some critics call the charter invasion of New Orleans a challenge to democratic values. Writing about New Orleans in a new book, an education team leader for the Center for Community Change in D.C., Leigh Dingerson, says Louisiana school authorities have “opened a flea market of entrepreneurial opportunism that is dismantling the institution of public education in New Orleans.”

But most educators and parents here are not taking sides in the ideological war over charters. An October teachers union report warned against “destructive rivalry” between regular and charter schools. Christian Roselund, spokesman for the United Teachers of New Orleans, said “the jury is still out” on how schools of any kind are going to perform. Regular schools are changing, too, and some state officials want to give tax-funded vouchers to help students attend private schools. Eighty-three percent of New Orleans public school families have low incomes.

Before the flood, New Orleans usually ranked near the bottom nationally in reading and math. This spring, results from the Louisiana Educational Assessment Program tests show modest gains for schools here, but overall achievement is still poor. There is no data yet on how charters compared with regular schools. About a third of fourth- and eighth-graders flunked the latest state tests. Sixty percent of sophomores failed in their first try at English and math tests that they must pass to graduate.

National charter leaders said they had not planned a giant takeover after Katrina. They said they just wanted to help. “Charter educators and friends took games and books and organized dozens of small classrooms while the national government scratched its head over what to do,” said Jeanne Allen, the president of the pro-charter Center for Education Reform, based in D.C. The Knowledge Is Power Program, one of the most successful nonprofit charter groups, was running a New Orleans school before the hurricane and managed to start a school for flood evacuees in Houston within six weeks.

The U.S. Education secretary, Margaret Spellings, reacting to the emergency, released $20 million in federal funds set aside for charters. Charter groups and teachers, eager for a chance to make a difference, rushed in.

Among the first arrivals was Ben Kleban. The former financial planner, who switched to high school teaching in Philadelphia because he “hated” his life, had planned to start a school in New York. But he detoured to Louisiana.

At age 27, he founded the New Orleans College Prep charter school, ushering in sixth-graders to a refurbished school building in the Central City part of town.

Before Katrina, Louisiana had passed a law friendly to charters, so Mr. Kleban and other charter founders faced few obstacles. A former Chicago and Philadelphia schools chief, Paul G. Vallas, was hired last year to run the Recovery School District, which splits responsibility for regular public school oversight with the Orleans Parish School Board. Mr. Vallas was allowed to add charters to his district. Charters “helped us get back in the game” quickly, he said, because the new schools could tap federal funds and private foundations.

Some new charters report strong results. The KIPP Believe College Prep and KIPP McDonogh 15 School for the Creative Arts more than tripled math scores and more than doubled reading scores of their fifth-graders last year, according to KIPP’s data.

Others have not performed nearly as well but continue to express faith in their ability to turn the schools around. “The main difference is that most of the charters have the freedom to change, to get better, to hire the people they need to make the school better,” KIPP’s director of operations in New Orleans, Jonathan Bertsch, said.

Mr. Vallas said he is pushing regular schools to innovate, giving them more autonomy and longer hours. He said he doubted that other cities will have as high a proportion of children in charters as New Orleans. Resistance to charters elsewhere, he said, is too strong.