Peer pressure is forcing some pupils to pretend not to work by missing classes but making up the learning in their own time during evenings, weekends, and holidays.
"It creates a real problem for schools," said Professor John MacBeath, director of the Quality in Education Cetnre at Strathclyde University at a conference promoting the growing study-support movement.
Rex Hall from the Tower Hamlets Education Business Partnership, said: "We have truants away all day coming back into study-support centres."
Claudine Field, co-ordinator of the Tower Hamlets Study-Support Project, said the centres tended to attract back into learning the truants who often hang around the school gates all day long waiting for friends who still attend lessons.
The phenomenon is being seen as a sign of the success of such centres - with their emphasis on "learning not teaching" - in attracting disaffected pupils, many of whom truant from formal lessons because of low self-esteem and underachievement.
There are believed to be around 300 such centres in the country, with numbers expected to hit a thousand by the turn of the century. Most are inspired and part-funded by a Prince's Trust initiative, which spends around a million pounds a year as "seedcorn" money to attract other investors, although some are largely financed by local authorities or Training and Enterprise Councils.
Now a three-year project is under way in Tower Hamlets, Sandwell and Merseyside to discover how well centres are doing.
Professor MacBeath, who will run the project, will look at changes in schools running study-support activities, including the number of exam passes, truancy and exclusion rates. He will also monitor the self-esteem of pupils involved in study support in the belief that as this rises, so does achievement in formal schooling. He said the ethos of such centres, with older pupils helping younger ones, gave teachers a chance to appear more approachable and get to know children in a more informal way.
Examples given at the day-long conference at the Queen Elizabeth II centre in London included that of Banbury School in Oxfordshire, which regularly has to turn pupils away from its library-turned-information centre at peak periods of the day.