This increases 10 hours each day at the weekend.
A new report shows 20% of first-year youngsters are spending 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 raise concerns about the future health of sedentary pupils.
The findings are part of a study commissioned by the Scottish Government to determine the impact of the Commonwealth Games on East End communities.
Working with NHS Health Scotland and Sport Scotland, the research will follow pupils from S1 to S5.
It aims to investigate the impact of having some of the main Commonwealth Games venues and events in the area.
Six schools were recruited to the study in three pairs: East End schools, schools in deprived areas and schools in more affluent areas.
Youngsters fill out a survey form - the Youth Physical Activity Questionnaire (YPAQ) - which asks them about their activities over seven days.
Questions cover four areas: sports participation, sedentary behaviour, active travel to school and time spent on homework.
The survey looks at the time young people spend online, watching TV or DVDs and playing computer games to tally up screen time.
A significant minority of pupils were found to spend a lot of time on-screen, particularly boys.
Some 20% of boys spend five or more hours on-screen each week day 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 than boys though pupils at advantaged schools spent more time on homework than any others.
One of the more deprived schools had very low numbers of pupils doing any homework with just 20% of boys spending their free time on homework.
Only 4% did any homework at the weekend.
However, researchers expect that as pupils get older they will spend more time studying at home.
Low expectations also had an effect on whether homework was completed or not with lack of parental support, lack of resources and poor conditions at home.
Analysis shows pupils who are entitled to free school meals do less homework no matter which school they attend.
For sports participation, it was found that children in advantaged schools have access to a wider range of sports than those in disadvantaged schools, both during the week and at weekends.
Researchers are now interviewing staff to find out why differences exist and what difference the Games has made to school sports.
Pupils from advantaged schools were more likely to take part in walking or hiking and cycling.
Football was the most popular sport for boys while running was the favourite for girls.
Active travel involves walking and cycling to school. Three out of five pupils were found to use active travel as a regular way of getting to school.
Researchers will now look at whether changes in the East End following the Games will have any impact on how pupils commute.
The second phase of the survey took place in May with the results expected at the end of the year.
Plans now are to discover whether sport participation can support 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.
"We encourage our young people to lead healthier, active lives through a number of health-promoting initiatives.
"Glasgow youngsters have even more opportunity to keep fit and enjoy sports both as part of the PE curriculum and as an after-school activity."