(CNN) - Mitt Romney, who is spending this week promoting a plan for America's public school system, spent Thursday morning defending his stance that smaller class sizes don't necessarily equate with better learning in schools.
Romney participated in an education roundtable at a charter school in Philadelphia the day after laying out his education plan to Latino small business owners in Washington. Romney's plan emphasizes school choice over other factors, including efforts to reduce the number of students in classrooms.
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Speaking of his time as governor of Massachusetts, Romney said he was frequently told that smaller class sizes would lead to better learning, but that certain studies advised otherwise.
"I came into office and talked to people and said, 'What can we do to improve our schools?'" Romney said at his Thursday event. "And a number of folks said we need smaller classroom sizes, that will make the biggest difference."
Romney went on to cite a study that showed no correlation between classroom size and performance, naming schools in Cambridge, Massachusetts as an example.
"The schools in the district with the smallest classroom sizes had students performing in the bottom 10%," Romney said. "Just getting smaller classrooms didn't seem to be the key."
Romney was pressed on his stance by a music teacher at the charter school who questioned the research Romney cited.
"I can't think of any teacher in the whole time I've been teaching, for 10 years, 13 years, who would say that more students would benefit them," Steven Morris, the teacher, said. "And I can't think of a parent who would say 'I would like my student to be in a classroom with a lot of kids with only one teacher.' So I'm kind of wondering where this research comes from."
Another teacher participating in the roundtable said unequivocally that he had too many students in his classroom.
"It's too large," the second teacher said. "It varies between classes, anywhere between 20 and 28. You can give more personalized attention to each student if you have a smaller class size."
Romney responded by naming a study from the McKinsey Global Institute, which is associated with the management consultancy McKinsey and Co. The presumptive GOP presidential nominee said the group's research compared class sizes in countries around the world, and showed the highest performing schools had class sizes the same as in the United States.
"So it's not the classroom size that's driving the success of those school systems," Romney concluded. More important, he said, was recruiting the best teachers and ensuring administrators have the correct priorities in leading schools.
While Romney cited one study to seemingly question the benefit of smaller class sizes, the majority of research does point to improved learning from smaller classes, according to Education Week.
One large study in Tennessee, which was conducted over a nearly thirty year period, showed students who were placed in a smaller-sized classroom made measurable gains, and performed better even when they were put back in larger classes.
Other studies show that smaller classes lead to higher achievement for minority students and students living below the poverty line.
Education Week notes that initiatives to promote smaller class sizes are costly, and sometimes lead to the hiring of under-qualified teachers.
Lis Smith, a spokeswoman for President Barack Obama's re-election team, responded to Romney's class size stance with a question: "What planet does he live on?"
"At his event today in Philadelphia, we saw Mitt Romney's vision for education and it truly tests commonsense," Smith wrote. "When confronted by teachers who know firsthand the benefits of smaller class sizes, Mitt Romney continued to insist - against all evidence - that larger class sizes are the answer to a good education. And he has even claimed that smaller class sizes 'hurt' education."