Pupils in countries that top international education league tables spend less time in after-school classes and individual study and more time in normal school lessons, according to new research.
The study, by the Organisation for Economic Co-operation and Development, suggests tuition and study outside school hours could even be associated with lower performance.
It compared the average percentage of pupils' science learning time spent in school lessons with countries' overall performances in the subject in the 2006 Programme for International Student Assessment (Pisa) survey.
Those countries where more than 70 per cent of learning time was in normal lessons, including Finland and the UK, tended to have the highest Pisa scores.
But Tunisia and Kyrgyzstan - among those with less than 45 per cent of learning time in normal science lessons - scored lowest.
Pupils in these countries spent more than four-and-a-half hours a week in after-school classes compared with the OECD average of nearly two-and-a-half hours.
The research concludes that this investment does "not necessarily" pay off.
"Learning time spent in after-school lessons and individual study is negatively related to performance," the report says. "Of course, this might be because students who attend after-school classes do so for remedial purposes, rather than to enhance their school studies.
"Still, across countries, findings show that students tend to perform better if a high percentage of their total learning ... is spent during normal school hours in a classroom."
The study comes as education secretary Michael Gove highlighted the proposed longer opening hours of the Norwich Free School - one of the free schools he is encouraging - which plans to open six days a week with only a four-week summer holiday.
John Bangs, who sits on the OECD trade union advisory committee, said: "This study shows out-of-school classes can't compensate for what schools offer.
"The idea that you can catch up after school is a misnomer. It shows the incredible added value that kids get from learning in a group, in a social institution."
UK's poor show
The UK has performed poorly in an international league table showing the proportion of disadvantaged pupils who "beat the odds" and succeed at school, according to the OECD.
A report published as The TES went to press last week showed it was placed 39th in a table of 66 countries based on Pisa results.
Asian school systems in Shanghai, Hong Kong, South Korea, Macao and Singapore were at the top.