Eric Hanushek, professor of economics and public policy at the University of Rochester, New York, and Caroline Hoxby, professor of economics at Harvard, produced evidence from experience in the United States to show smaller classes do not necessarily lead to improved pupil performance.
Some US studies even suggest that cutting class sizes results in falling standards, they said.
Speaking at a debate organised by The TES and the Centre for the Economics of Education, Professor Hanushek said that since the 1960s there had been no improvement in US pupil test scores, despite a threefold increase in school spending per pupil and a cut in pupil-teacher ratios by a third. International studies showed similar results, he said.
Professor Hoxby, whose own research oncluded that reducing class sizes resulted in only a marginal academic improvement, said: "The public is willing to spend money on schools but that energy has not resulted in improvements. The question is how you make these resources more productive."
Teaching standards have the biggest impact on pupil performance, according to Professor Hanushek. He said that three years of good teaching can eliminate the effects of serious social deprivation.
He criticised the Government's plans to introduce performance-related pay, saying ministers should not have introduced it nationwide before gathering evidence on its impact in schools.
Responding to a question from Peter Robinson of the centre-left Institute for Public Policy Research, Professor Hanushek also said that accurate, value added measuring of teachers' performance was vital if the reforms were to be fair and effective.