To a group of a dozen or so listeners that included Gamble, school administrators and two teachers, Romney recalled how, as governor of Massachusetts, he studied what boosts test scores and what doesn't. His team analyzed results at schools in 351 cities and towns in his home state and found no correlation between the number of students in a classroom and performance.
"As a matter of fact, the district with the smallest classrooms, Cambridge, had students performing in the bottom 10 percent," Romney told the Bluford group. "So just getting smaller classrooms didn't seem to be the key."
He also cited a study by the McKinsey Global Institute, research arm of the global management consulting firm McKinsey & Co., that found teachers' and parents' involvement mattered most.
In essence, Romney was differing with one of the givens of modern American elementary and secondary education: that smaller class size produces better results.
That was when two teachers at the table spoke up, albeit in civil tones.
"I heard your position on class size and testing," music teacher Steven Morris said. "I can't think of any teacher in the whole time I've been teaching, over 10 years, 13 years, who would say . . . that more students would benefit [from larger classes]. And I can't think of a parent who would say, 'I would like my teacher to be in a room with a lot of kids and only one teacher.' "
Morris then turned to Ronald Benner, who teaches technology at Bluford, and asked if he would want more students in his class.
"No," Benner replied.
Another person at the table, David Hardy, chief executive of Boys Latin of Philadelphia Charter School, jumped into the discussion. Hardy, whose school is highly regarded for academics, pointed to a University of Tennessee study that found small class size crucial to pupils' success, especially in the early grades.
Classes in Philadelphia public schools run as high as 33 students. Universal charter schools try to cap classes at 22 students each.
The setting of Romney's 90-minute morning visit was striking: he was bringing his Republican campaign into overwhelmingly Democratic territory, staging a media event in a school where students pass through hallways adorned with a portrait of President Obama (along with that of astronaut Bluford, who grew up in West Philadelphia).
Romney's comments on class size drew mixed reactions from education experts contacted later in the day by The Inquirer.
Jonathan Supovitz, associate professor at the University of Pennsylvania Graduate School of Education, said he was not familiar with the McKinsey work Romney had cited. The Tennessee research, he said, was "very rigorous" and tracked the progress of many students in classes both larger (22 to to 25 pupils) and smaller (13 to 17 pupils).
"Absolutely the results . . . showed that the smaller class sizes produced improvement in early learning," Supovitz said. "And it was particularly effective for minority students."
But questions remain about whether smaller classes are more cost-effective than spending money on teacher training or other changes, he added.
Jerusha Conner, an education expert at Villanova University, said studies suggest reducing class size has "negligible to modest effect" on test scores but may produce longer-term benefits - such as higher graduation rates.
Universal is in its second year of operating the Bluford site, formerly a public school. Students at Bluford failed to make state standards for Adequate Yearly Progress, but the charter school's officials said test scores and attendance have improved since they took over.
Gamble told Romney that the No. 1 job of the next president is to heal the country's racial divide. Blacks are struggling in part because of the long history of discrimination, Gamble said.
"Where there was a time when it was against the law of the country for people of African American descent to even read or write, it is even more important today that we discuss education for the African American community," he said.
Gamble said the Romney campaign, which on Wednesday unveiled its educational platform, had reached out to Universal to set up the visit. He said company officials agreed to host the candidate to encourage a conversation about education.
"This is not an endorsement," Gamble said. "It's a meeting of the minds. He was interested in some of the work that we are doing."
Gamble said he does not know whom he will vote for in November and generally tries to avoid taking sides. "I always look at the Democrats and the Republicans as sort of like the Crips and the Bloods," he said. "My interest is what is going to happen in education."
After the roundtable in the library, Romney went to some classrooms, shook hands with children and sat down for a lesson on literary devices, such as alliteration.
After a few minutes' work, Romney and a group of children came up with a sentence - "Dogs drive Douglas dizzy" - to illustrate the lesson.
"I think my favorite class was English," he told the children, adding that it had helped him succeed in business.
He also dropped in on a music class, where second-through-sixth graders sang Kirk Franklin's "I Smile." The candidate smiled and chatted but did not clap along.
"You just sang a song about smiling," Romney said. "You're all smiling, right? Smile! Oh, that's great."
Contact staff writer Miriam Hill at 215-854-5520, firstname.lastname@example.org or @miriamhill on Twitter.
Inquirer staff writer Kristen Graham contributed to this article.