SAG HARBOR — The opening of the Hamptons’ first charter school on a country road here lined with sprawling summer estates has spurred improvements in at least one local public school, which was forced for the first time to compete for students.

Founded by a mother seeking an educational alternative for her autistic son, the Child Development Center of the Hamptons was the first school to focus on special needs children in the area. For the Shelter Island school district, which has only one school serving 250 students from kindergarten to 12th grade, the arrival of the charter school set into motion a competition for students and resources. Charter school advocates — who include President Bush, who is scheduled to visit a New York City charter school today — say that such competition will help to revitalize the nation’s public schools.

“One of the things we hope charter schools do is to spur competition,” a spokeswoman for the State University of New York Charter Schools Institute, which authorized the school, Cynthia Proctor, said. “In this case it looks like that this is what happened.”

The slightly quirky Center for Child Development stood in stark contrast to the stately Shelter Island School. In an isolated community where there had never before been a choice in schools, to some parents, the mystique of the new school was irresistible.

A Shelter Island school board member, Donald Bindler, described the interest in the new charter school to the ripple of excitement when a new restaurant opens.

“On a small island like ours, when a new restaurant opens in town, everybody says, ‘Oh wow, we’ve got to go try it.’ I think that was some of the reaction here,” he said. “People said this is this new alternative education for my children, and it’s still free.”

On a recent morning at the charter school, a small child ran at full speed through the halls pushing his occupational therapist on a skateboard, sending her flying into the school’s front desk in a hail of laughter. The first grade class did a yoga performance for a group of visitors as a teacher read a story that corresponded with each move. An older student greeted administrators with a hand slap.

Even the barn-shaped structure of the building reinforces the whimsical, informal sensibility of the school: the walls of the classrooms are designed to be detachable so that they can be rearranged into different formations within the building.

The Shelter Island School, a three-story brick behemoth, is the largest building on the island. Its pale pink and sea green cement brick hallways are lined with lockers and posters for the upcoming performance of the spring musical, “Les Miserables.” Children filing through the halls carting laptops greeted their superintendent, Sharon Clifford, with a demure, “Hello, Mrs. Clifford.”

The school performs consistently well on state tests, and little had changed there in years, until the charter school opened. For each student that goes to the Child Development Center this year, according to numbers provided by the state, the Shelter Island school district must pay the charter school $25,000 — an amount derived from the district’s annual per pupil expenditure. After the charter school first opened, 22 students left from the district in one year, leaving Shelter Island to pay nearly half a million dollars of its $8 million annual budget to the Child Development Center, not including the costs for transporting its former students to the new school, school board members said.

It was enough to prompt changes at the Shelter Island School, which added new programs like balanced literacy for its younger students, writing and college seminar classes for the older students, and expanded its computer program. Ms. Clifford was hired after the former superintendent passed away, and the school improved its special education offerings.

Soon, the district had succeeded in bringing back more than half of the students that had left.

“All those changes made people feel that they wanted to be here again,” Ms. Clifford said.

The charter school’s executive director, Donna Colonna, and the assistant director, Richard Malone, said their school’s alternative environment provides an important haven for children unlikely to thrive in the area’s more traditional schools. At the Child Development Center, each child has an individual education plan, whether they have special needs or not, and students are often organized by ability instead of grade.

Several school superintendents from the surrounding area, including Shelter Island school board members, said they welcome the charter school and its role in offering school choice to the Hamptons’ year-round residents. The Bridgehampton superintendent, Dianne Youngblood, said her district’s finances were threatened briefly when a group of eight parents decided to send their children to the charter school. She quickly met with the parents to discuss their concerns, and five decided to come back.

The Shelter Island school board has been holding meetings with state legislators, where they have asked them to rethink how charter schools are funded.

“It’s not a matter of who is or who isn’t at the charter school,” Ms. Clifford said, noting that parents can decide to move their children at any time during the year. “It’s a matter of how does one budget.”

For the first time this year, the state budget is including transitional aid funding to offset the costs to districts that send students to charter schools. Ms. Clifford said she has estimated that her district will get about $200,000.

The vice president for School Fiscal Accountability at the SUNY Charter Schools Institute, William Lake, says “The revenue that a district receives per student is always greater than the amount the district pays to the charter school,” Mr. Lake said. “So the budgetary impact on a district is always less than the enrollment impact.”

Still, the Shelter Island administrators are keeping an eye on the competition.

“Five kids for some reason could decide, ‘We’re going to the charter,'” Mr. Bindler said. “It could change on a dime.”