The already packed schools in Lower Manhattan and parts of the Upper East Side are facing increased crowding as New Yorkers move into the new apartments going up in those areas.

Public school enrollment is expected to fall by almost 9% citywide by 2014, but some of the wealthiest neighborhoods in Manhattan can expect to see a 25% rise in the number of students, according to a report by the city’s School Construction Authority.

The bulk of growth is expected to be concentrated in the Lower Manhattan portion of District 2, where the combination of rising birth rates and the construction of apartment buildings already has created an increased demand for public school slots.

“I don’t know whether it’s because housing costs are so incredibly high that people can’t afford private school,” a sociology professor at the CUNY Graduate Center, Philip Kasinitz, said. He said the improved public schools are attracting more middle-class parents, and that parents in Manhattan tend to wait longer to have children than in other boroughs.

“We’re growing at an incredible rate down here,” the district manager of Community Board 1 in TriBeCa, Paul Goldstein, said. “We see from our local schools that they are getting overcrowded, we see from Little League that they’re growing every year … at the seaport we have fishmonger buildings being converted as we speak.”

At the same time, the city is projecting a more than 30% drop in enrollment in some of the city’s poorer neighborhoods, like Bedford-Stuyvesant and Brownsville. Also expected to decrease is enrollment on the Upper West Side, by 6.4%.

TriBeCa and Battery Park boasted Manhattan’s highest birthrate in 2004. More than 15,600 new apartments are in the pipeline or have already come on the market in the area since 2000.

The well-regarded P.S. 234 on Chambers Street in TriBeCa already was forced to cut its pre-kindergarten program because of overcrowding, and next year it is slated to lose its science room to make way for more classroom seats, parents said. The school, which serves children on the West Side below Canal Street, was at 113% capacity in 2004.

The School Construction Authority’s annual projection reports are used in part to determine which districts will receive increased funding for capital projects like building new schools. The most recent report examines projected student registration between 2005 and 2014. The city uses birth rates and the migration patterns of residents into and out of the city to estimate the school-age population.

The biggest drops in enrollment are slated for Districts 13, 14, and 16 in Brooklyn, which include Greenpoint, Williamsburg, Fort Greene, Bedford Stuyvesant, Ocean Hill, and Brownsville. Each district is expected to lose more than one-third of its school age children.

Mr. Kasinitz attributed the drop to a variety of factors, including an aging population, gentrification, and religious schools. “The last Polish immigrants to New York showed up right after Solidarity and they’re done having kids,” he said about the Greenpoint section of Brooklyn. “Yuppies are moving in and they don’t have many kids.”

Nearby Williamsburg boasts one of the highest birthrates in the city, but many of the area’s chasidic Jewish families send their children to religious schools, leaving empty classroom seats in public schools.

Gentrification in neighborhoods like Fort Greene and parts of Bedford Stuyvesant also can mean a decrease in school age children.

“Rooming houses are being turned back into one-family houses, Mr. Kasinitz said. “The people who move in take up more room than the people who moved out.”

Angry parents packed into a Community Board 1 meeting in Lower Manhattan last week to ask Deputy Mayor Dennis Walcott why the mayor recently backed down on his promise to build a new kindergarten to 8th grade school on Beekman Street and a planned annex for P.S. 234.

Last month, Mayor Bloomberg announced that the projects would be put on hold if Albany didn’t come through with billions of additional school funding dollars as part of the Campaign for Fiscal Equity lawsuit.

“Parents have been asking that this be fixed for a long time,” the vice chairwoman of the PTA at P.S. 234, Mariama James, said. “Buildings are going up all over and children and families come with these building and we need someplace for their children to be educated.”

A Department of Education spokeswoman, Marge Feinberg, said the schools are a “top priority.”

“But without appropriate capital funding from the state we cannot go forward on many of our projects,” she said.

Because the reports are used in part to determine funding for capital projects like building more classroom seats, some elected officials are concerned that their districts will be shortchanged.

The council member who represents Fort Greene, Leticia James, questioned the estimated 30% decrease in her district. She said the proposed Atlantic Yards project is expected to bring an additional 7,500 apartments to the area.

“I just don’t think that there’s going to be a decrease,” Ms. James said. “I just don’t get it.”