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 questioning the benefit of smaller class sizes, the majority of research does point to improved learning from smaller classes, according to Education Week.

http://www.edweek.org/ew/issues/class-size/

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."