But giving more money to the district through a "quick fix" like the $2-a-pack cigarette tax would not solve its problems, he said.
District officials dismissed the four-page report.
"It adds noise to a conversation that should be about facts," district spokesman Fernando Gallard said. "You cannot apply simple math to a complex problem. That is what they are trying to do."
The state House had been scheduled to vote Monday on a bill that would let Philadelphia impose the tax on cigarettes, but indicated Thursday that it would stay out until September.
School District Superintendent William R. Hite Jr. and city officials want the House to approve the measure before Aug. 15, when layoff notices would have to go out.
The district has been counting on the money to help plug a $81 million deficit. Absent the funds, Hite has said, he will have to lay off 1,300 people and boost class sizes to 40 or more.
Although the Commonwealth Foundation's report was posted on its website Thursday, Benefield said it played no role in the legislators' decision to delay the vote. "That was decided before we released it," he said.
Benefield said the study was prepared in anticipation of the expected Monday vote.
"It was to help inform the debate," he said.
Gallard had a different take: "They would be providing misleading information to legislators on a very crucial vote on an important subject."
According to the foundation, the amount the district spent per student grew from $9,299 in 2002-03 to $14,361 in 2012-13. Some other districts spend much more. Lower Merion, for example, spent $26,808 per student in 2012-13.
During the period reviewed, enrollment in district schools dropped 25 percent while the number of classroom teachers was down 6 percent.
Benefield said the foundation's analysis went only to 2012-13 because that information was available from the state Department of Education's website. The report's conclusions do not reflect the financial woes of the school year just ended that caused cutbacks of counselors, nurses and teachers.
The study also said that despite past funding increases, results from the National Assessment of Education Progress - popularly known as the "Nation's Report Card" - showed scores were flat between 2009 and 2013. Philadelphia's fourth and eighth graders performed below the average for other large urban districts in reading and math.
Over the period reviewed, the foundation said, the district's student-teacher ratio declined. In the 2002 school year, there were 19.5 students for every teacher; 10 years later, it was 15.6 students per teacher.
Based on that analysis, Benefield said, the district's warnings that class sizes would grow without funds from the cigarette tax were exaggerated.
The district said that was not so.
"We have nowhere to go except to increase" class size, said Matthew F. Stanski, the district's chief financial officer. Benefield's talk about exaggeration "is just not true," he added.
Stanski said Benefield's analysis was misleading because it lumped programs that have smaller class sizes - such as special education and ESL - with regular classes. Standard classes in kindergarten through fourth grade have 30 students, and grades 5 through 12 have 35.
Sharon Ward, director of the left-leaning Pennsylvania Budget and Policy Center, said Philadelphia had to suffer a disproportionate share of the cuts imposed on school districts in 2011, and that current state funding for basic education is $245 million below 2010-11.
"When state funding increased, the performance of Philadelphia's schoolchildren improved," she wrote in an e-mail. She said that the percentage of students who tested at grade level or above in reading more than doubled between 2002 and 2010, and that the percentage in math nearly tripled.
While all this was going on, more than 50 students, parents, and teachers rallied late Friday afternoon outside Gov. Corbett's offices in Center City to protest inaction on the cigarette tax, chanting "S.O.S. - save our schools," and holding signs that read, "Send Corbett Packing," and, "Don't Destroy My Future."
Kensington Creative and Performing Arts junior Nazir Nimson said he was protesting because he was a victim of the budget cuts.
"All kids need education," he said, adding that the House's extended recess was "disrespectful."
Inquirer staff writer Lydia O'Neal contributed to this article.