On the first day of class here today, 366 students showed up at the door, boosting the student body by nearly 20 percent in a year. Another 30 students are still trying to get in, which means that, for the first time in its 72-year history, this Catholic school needed a waiting list.

Everything and nothing has changed at Messmer. Teenagers today fumbled with their lockers, searched for new homerooms and misplaced class schedules. What is different is that, for nearly half of the students attending this private religious institution, taxpayers -- not parents -- will pay the bill.

The familiar sounds and monotonous hum of a new school year at Messmer represent the extraordinary debut of the nation's most ambitious effort to retool urban education by allowing poor children to attend religious schools using state-funded vouchers.

"Welcome to the beginning of a new era in education in Milwaukee," said Gov. Tommy G. Thompson (R) in an address to students today in Messmer's auditorium. "Instead of busing our kids all over town to a public school that just doesn't measure up, we're going to give parents the chance to send their kids to a school right down the street," Thompson said. "And if that school is private, so be it."

Messmer's growth spurt stems largely from a ruling by the Wisconsin Supreme Court in June that cleared the way for more than 110 parochial and private, nonsectarian schools to receive public stipends -- or vouchers -- to teach students previously enrolled in Milwaukee public schools. Rejecting an appeal by opponents of school vouchers, the court ruled that the program violates neither the state's constitution nor the First Amendment, barring laws that promote religion.

At $4,900 annually per student, the city's voucher program will pay for as many as 15,000 low-income schoolchildren to attend private schools. Since the money comes from the public school system's budget, civil rights groups and teachers unions have opposed the program, arguing that the subsidy will siphon resources from already troubled public schools.

City schools opened last week, and officials say that enrollment, based on applications, has increased slightly from 103,000 pupils last year. Still, they say it typically takes a few weeks for administrators to get an accurate head count, and state officials estimate that nearly 6,000 children will participate in Milwaukee's voucher program this year.

People for the American Way, a District-based civil rights organization, estimates that vouchers will cost Milwaukee's public schools more than $29 million in this school year alone.

"It's a recipe for flight from the public schools," said Elliot Mincberg, an attorney and vice president of People for the American Way, which today appealed Wisconsin's decision to the U.S. Supreme Court.

Messmer, among the first parochial high schools to open this year, is considered ground zero for Milwaukee's voucher enterprise, perhaps the city's primary beneficiary of the new program. Located in a toughened, overwhelmingly black neighborhood on the city's north side, Messmer draws heavily from the surrounding community.

And while Wisconsin's voucher program does not require children to attend neighborhood schools, proponents believe it will reduce long cross-town commutes.

Academically, the Catholic school has fared significantly better than most of the city's high schools. Messmer's graduation rate is 98 percent -- more than double the city's rate -- and 85 percent of its graduating seniors go on to college.

The school does not require school uniforms, but it is demanding. Students cannot chew gum, wear caps or swear, and the school's president, Brother Bob Smith, a Capuchin friar, can expel anyone he wants without a hearing. Expulsions still are rare, however. The absentee rate is 3 percent daily, on average.

But before they can settle in, the new children will need to find chairs. Expecting an influx of students, Ann Szekely grabbed a few extra chairs from another classroom. It still wasn't enough; 18 students crowded into her classroom today, and some had to sit on the floor until a few more desks could be retrieved.

"I've never had so many kids wanting to get into my class before," said Szekely, a teacher for six years, the last four at Messmer. "Usually they're trying to get out."

But most faculty members here are enthusiastic about vouchers and Messmer's growth. "It's exciting to me," Szekely said. "I feel like I'm on the cutting edge of education."

Aisha James, 16 and opinionated, attended a city school last year and is glad her mother suggested she transfer this year.

"You can learn better here," she said. "People listen to the teacher. At Madison [a public high school], they don't teach you [anything]. The kids fight all the time, and the principal don't try to help people. If there's a problem, he would just suspend you."

Deseree Gordon said she couldn't imagine sending her two daughters to a public school now that vouchers are available. "There's just too many kids in gang trouble or involved with violence," she said. "The kids can actually learn something when they don't have to deal with all that."

Wisconsin began experimenting with school vouchers in 1990, but legal appeals blocked the state's plans to expand the program to religious schools in 1995. The state's efforts represent the largest voucher program in the country and the only one where children are allowed to attend religious schools.

Milwaukee Mayor John O. Norquist (D), who like Thompson was an advocate for the city's use of school vouchers, appeared with Thompson at Messmer today to commemorate the shift in educational policy. He acknowledged the acrimonious and lingering resentment over the issue in speaking to reporters afterward about his appearance.

"I don't think it would have been appropriate to celebrate school choice at any of the public schools," Norquist said. "There are still some pretty raw feelings."