For years, teachers and parents have insisted that smaller class sizes are crucial to children's educational success.
On Thursday, Mitt Romney visited Philadelphia and politely said they were mistaken.
And on Friday, passions erupted among partisans and professionals, from city classrooms to City Hall to Cherry Hill.
"Out of touch with reality," Mayor Nutter fumed about the presumptive GOP nominee.
"Just plain wrong," said Steve Baker, spokesman for the New Jersey teachers' union.
It's a tenet of modern American elementary and secondary education: smaller classes produce better results. But is that accurate?
It's certain that smaller classes are enormously popular - not just with parents and educators, but with legislators. In recent decades, a Brookings study found, at least 24 states have acted to require or encourage reductions in class sizes.
But here's the problem: It's expensive.
Having smaller classes means hiring more teachers and backing them with more support services. That's particularly difficult in a tough economy. Such investments, in places where they've been made, have been justified by the belief that smaller classes effectively increase learning and achievement.
In 2011, Brookings sought to examine the large body of studies on class size and to identify the most credible.
On top was the Student Teacher Achievement Ratio, or STAR, conducted in Tennessee in the late 1980s. It found that classes with 15 students achieved more than classes with 22 - the smaller classes gaining the equivalent of three extra months of instruction.
Studies in Texas and Israel found smaller gains. Other rigorous studies found mixed results in California and no effects in Florida and Connecticut.
"Conclusions have to be tentative," the Brookings study said. "But it appears that very large class-size reductions, on the order of magnitude of 7-10 fewer students per class, can have significant long-term effects on student achievement."
Teacher Aileene Halligan didn't need a study to confirm what she sees every day.
Until recently, she taught eighth-grade Spanish at Ethan Allen Elementary in Northeast Philadelphia, where her classes held as many as 38 students.
"You're praying that five kids are going to be absent each day," she said. "I couldn't get to the desks, it was so crowded. . . . Worse was the faces of the kids, looking at you like, 'Please, get to me today.' "
Now Halligan teaches ninth- and 10th-grade history at Kensington Urban Education Academy, a public high school, where most of her classes have 30 students and one has 19. The larger classes are manageable, "but it's always the smaller the better."
Field trips are easier. Donations and resources go farther. And most important, she's able to spend more time with individual students.
"To say class size doesn't matter," she said, "is a real slap in the face."
Efforts to reach Romney Pennsylvania campaign officials were unsuccessful Friday.
On Thursday, Romney visited a West Philadelphia elementary school and suggested that class size mattered little to achievement. Teachers in the room immediately respectfully told him he was full of soup.
Talking to a dozen or so roundtable guests at Universal Bluford Charter School, named for astronaut Guion Bluford, Romney said quality teaching and parental involvement were the keys. He recalled how as governor of Massachusetts, he studied what does and does not boost test scores.
His staff analyzed schools in 351 municipalities - and found no correlation between performance and class size, he said.
"As a matter of fact, the district with the smallest classrooms, Cambridge, had students performing in the bottom 10 percent," Romney told the group. "So just getting smaller classrooms didn't seem to be the key."
He cited a study by the McKinsey Global Institute that found teacher and parent involvement mattered most.
Classes in Philadelphia public schools run as big as 33 students. Statewide, the average is 21. The national average is 20 in elementary schools and 19 in high schools, according to the federal Education Department.
From seventh grade through graduation, Romney attended the private Cranbrook Schools in Bloomfield Hills, Mich.
Classes there are small, according to statistics the school provided Friday.
In pre-kindergarten, classes have 15 students, taught by two teachers and an assistant teacher. Kindergarten classes have 18 students and two teachers. Grades one to five are limited to 18 students per class, and grades six through eight typically have 15 or 16 students. Classes in ninth through 12th grades are capped at 18 students.
"Class size does matter," insisted Penny Nixon, the Philadelphia School District's chief academic officer. And it matters especially, she said, in an urban district, where students are more likely to need extra support.
It's true that research has come down on both sides, Nixon said. But in her time as a middle school teacher and principal, "the difference between 25 students in a class and 33 students in a class is huge."
Size is so important that even though the district is in a financial crisis - cutting $700 million from its budget over the last year and facing a $218 million shortfall next year - it's going ahead with plans to reduce class numbers in kindergarten through third grade. The goal is to have no more than 25 students in those classes.
Students had their own opinions Friday.
Outside Cherry Hill High School West, Louis Stewart, a 17-year-old junior, said he tended to think class size wasn't so important.
"I would say it has more to do with the teacher," he said. "Some teachers are more involved with their students than others. But it would be harder to do that with a larger class."
Julia Lanuez, a 16-year-old junior, said class size mattered. "When it's a smaller class, it's more personalized learning and easier to meet everyone's needs."
Elsewhere in Cherry Hill, Kwame Morton, principal of Joyce Kilmer Elementary School, cautioned that some research showed small classes alone will not yield quality results. "You have to look at lower class size along with teacher efficiency," he said.
On Friday, Nutter took part in an Obama campaign conference call to criticize Romney's views.
"I'm not sure what universe he's operating in, but we certainly know in Pennsylvania, every parent knows, every second-grader knows, that smaller class sizes are preferable," the mayor said.
During the call, Erin Thesing, a first-grade teacher at Mann Elementary charter school in West Philadelphia, endorsed Obama's education plans and described her difficulties with larger classes.
At the start of the school year, her class roster had 25 names - but 30 students showed up, forcing her to squeeze in desks, tables, and chairs. Every day, parents demanded: Why must my child sit so far from the board? When will the class shrink?
Eventually, her class was reduced to 21 students, "an absolute dream" that enables her to give one-on-one attention to students. And, she said, she no longer steps over chairs and boxes.
"The more personal attention I can give my students," Thesing said, "the better."
Inquirer staff writer Kristen A. Graham contributed to this article.