The finding contradicts a recent report by the school inspection body, Ofsted, which says the quality of teaching is more important than the number of children in the classroom.
Academics from the University of Nottingham say that smaller classes are needed for all age groups, but Ofsted argues that class size is only important for infant pupils.
The authors of a report commissioned by the National Association of Head Teachers dismissed the inspectors' findings as invalid and unreliable.
Ofsted representatives were sent in to schools to look at the quality of education and not at class size, they said.
The research, led by Professor Christopher Day, said that large classes had an effect on pupils' behaviour and on the ability of schools to improve, as well as on teachers' workload and stress.
Classes of between 15 and 17 pupils led to higher standards and more positive attitudes to learning, particularly among pupils from disadvantaged backgrounds, Professor Day said.
The professor's report came as government figures revealed that four out of 10 primary pupils were being taught in classes of more than 30 and that the numbers were rising.
Professor Day said more research was needed into the effects of large classes on children in this country.
He added: "Rising class size has an effect on the quality of opportunity for learning and it has a potentially damaging effect on the quality of teaching.
"I think there is too much weight given by the government to Ofsted reports which are about evaluating the quality of the school and not about investigating the effects of class size."
A spokeswoman for Ofsted said its research was reliable, and that it had been based on evaluations of 200,000 lessons.
"The inspection findings confirmed that the effectiveness of teaching has a more significant influence on pupils' achievements than class size alone," she said.
David Hart, general secretary of the NAHT, said that Ofsted should look again at its findings and that ministers should acknowledge that they were flawed.
"Pupils in smaller classes, misbehave less, they participate more, they spend more time on tasks and they interrupt less," he said.
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