A new report shows 20 per cent of boy pupils spend a minimum of 25 hours a week playing computer games, watching DVDs and online.
Researchers said the results of the GoWell in the East End study, which examines communities in Glasgow, raise concerns about the future health of sedentary pupils.
Six schools were recruited to the study in three pairs: East End schools, schools in deprived areas and schools in more affluent areas.
A significant minority of pupils were found to spend a lot of time on-screen, particularly boys. Some 20 per cent of boys spend five or more hours on-screen each weekday, while one in eight spend 10 or more hours on-screen at the weekend.
Boys spend more time playing computer games while girls spend more time watching TV or DVDs.
Pupils at the more advantaged schools were more likely to spend time online at the weekends.
It was also found that girls spend longer on homework, though pupils at advantaged schools spent more time on homework than any others.
One of the more deprived schools had very low num-bers of pupils doing any homework, with just 20 per cent of boys spending their free time on homework. Only four per cent did any homework at the weekend.
However, researchers expect that as pupils get older they will spend more time studying at home.
Plans now are to discover whether doing sport can boost studying, homework and educational success.
A Glasgow City Council spokeswoman said: "It's important that young people get the chance to participate in all kinds of physical activity, and schools across the city offer a wide range of activities for people of all fitness levels."
The findings are part of a study commissioned by the Scottish Government to determine the impact of the Commonwealth Games on East End communities.