Only hours before a Democratic gubernatorial candidates' forum Wednesday on education, a left-leaning Harrisburg think tank released a report outlining the harm done by the state's failure to sufficiently fund public schools.

"We want to contribute to that debate," Sharon Ward, director of the Pennsylvania Budget and Policy Center, said during a news conference at City Hall.

The report said cuts to state funding for schools made after Gov. Corbett took office three years ago have hurt all districts but have disproportionately harmed those with large numbers of low-income students.

Before the state's school funding formula was scrapped in 2011, districts with large numbers of low-income students received additional money from the state.

"The scale of the funding cuts here in Philadelphia and other low-income districts across the state is simply unprecedented," Ward said. "Since 2011, Philadelphia has had almost a $294 million reduction in state funding. This is important because a little bit less than 12 percent of students in the commonwealth experienced 35 percent of cuts that were implemented."

Philadelphia had the largest cut of all the districts in the state. The center calculated that it lost $1,351 per student. Chester Upland lost $1,194 per student, and some wealthy suburban districts lost from $36 to $59.

Corbett, a Republican, has forcefully defended his education record. The budget he has proposed for the fiscal year starting July 1 provides an increase of $397.5 million for public schools. The center said that amount plus modest increases in the last two years would restore only 46 percent of the 2011 cuts.

Timothy Eller, a spokesman for the state Department of Education, disputed the center's account and said Corbett had increased "the state's support of public schools by $501.7 million in 2011-12."

"It's extremely misleading for the Pennsylvania Budget and Policy Center to continue to push the false charge that Gov. Corbett cut funding for public schools; they, as well as many other advocacy groups, fail to mention that Pennsylvania's public schools lost more than $1 billion in federal stimulus funding, something everyone in public education knew was temporary and something Gov. Corbett had no control over," he said in an e-mail.

Marjorie Neff, principal of J.R. Masterman, a city magnet and the top-rated school in the state, said her school had lost 30 percent of its staff since 2011. "That is mirrored across the city," said Neff. "The reality is there aren't enough adults in schools in Philadelphia to do justice to the kind of education kids are entitled to."

Ward's report pointed out that as the state cut money for public schools, it also reduced corporate taxes and "diverted additional funding for alternatives to public schools," including charter schools and tuition-assistance programs for private schools.

The center found that while cuts in state funding were taking place, districts were grappling with rising costs they could not control, including pensions and charter schools.

To balance budgets, districts raised property taxes and laid off staff. The center said 20,000 education jobs have been lost across Pennsylvania since 2011, which slowed the state's economic recovery.

Ward's report calls on the legislature to enact a school funding formula to distribute state funds equitably and to consider measures to increase state funding for schools, such as taxing Marcellus Shale gas production.