As it prepares for large-scale school closings, the Philadelphia School District must brace itself for a painful process that probably won't generate much revenue, according to a report by the Pew Charitable Trusts released Wednesday.

The report, by Pew's Philadelphia Research Initiative, examined districts that have engaged in similar closings over the past decade, including Chicago, Detroit, Kansas City, Milwaukee, Pittsburgh, and Washington.

It concluded that short-term financial gains have "been relatively small in the context of big-city school-district budgets" - with real savings achieved only with mass layoffs.

"Longer-term savings are difficult to project," the report said.

The district has cited falling enrollment, outdated buildings, and budgetary concerns as the reasons it must close and consolidate schools, though officials have downplayed the savings they expect to realize.

Recommendations on closures are expected this month or early next month.

The district's enrollment has dropped 23 percent in 10 years, from 201,190 students to 154,482. Fewer school-age children and a boom in charter schools have hastened the decline.

Though it has closed a handful of schools and given others to charter organizations - which lease district space - the district still runs 249 schools, many of them old and in bad shape.

The average district school is 63 years old. One school, Francis Scott Key, was built in 1889.

Closures are likely to hit the city unevenly.

Though some neighborhoods have thousands of excess seats, others are full. Schools in the Northeast operate at 100 percent capacity, for instance; in the Southwest, schools are only at 55 percent capacity.

Recommendations for closures, consolidations and grade and boundary changes are expected from district staff in the next few weeks. After that, hearings will be held with a School Reform Commission vote to be held in February.

It is unclear when any school closures will be announced or occur.

In many cities, researchers found, vacant old school buildings, many of which are in tough neighborhoods, have proven to be a tough sell. As of this summer, there were 200 unoccupied and unsold school properties in the districts Pew studied - 92 in Detroit alone.

In Philadelphia, the district has 14 empty buildings on its hands, and a number of former Catholic schools already stand vacant.

Though closings have been better received in some cities than others, they appear certain to generate political fallout.

In Washington, anger over the way the closings were handled led in part to the mayor and schools chancellor losing their jobs; in Chicago, the process led to a new law dictating how school closings will play out in the future.

Researchers also found that school closings appear to have little long-term effect on student performance - though once a closure is announced students at a particular school seem to struggle, they often recover those losses.

District officials, Pew points out, have already studied other cities' school closing procedures and taken cues from their struggles and successes.

In Philadelphia, "they certainly have done a fair amount of outreach and they conducted a lot of meetings," said Larry Eichel, project director of the Philadelphia Research Initiative.

"But to some degree, the public was paying attention to the budget crisis, and it's hard to gauge how much impact their outreach efforts have had."

Eichel said the most difficult parts of the closure process are yet to come.

"This is going to be messy no matter how it's done," he said. "It's been messy everywhere. There's no silver bullet, no perfect way to do it."