An extra hour a day of television, internet or computer game time in Year 10 is linked to poorer grades at GCSE, a Cambridge University study suggests.
The researchers recorded the activities of more than 800 14-year-olds and analysed their GCSE results at 16.
Those spending an extra hour a day on screens saw a fall in GCSE results equivalent to two grades overall.
"Reducing screen time could have important benefits," said co-author Dr Esther van Sluijs.
The researchers analysed the habits of 845 pupils from schools in Cambridgeshire and Suffolk at the age of 14 years and six months.
The pupils heights and weights were recorded, and they had to wear a physical activity monitor for five days including a weekend.
They were also asked to complete a questionnaire detailing the amount of time they spent on:
- reading for pleasure
- physical activity
- watching TV
- playing video games
- non-homework time online
The researchers correlated the data with the pupils' GCSEs, taken the following year.
Pupils who did an extra hour of homework and reading performed better than their peers, while extra physical activity appeared to have no effect on academic performance.
On average, the 14-year-olds said they spent four hours of their leisure time each day watching TV or in front of a computer.
The researchers found an additional hour of screen-time each day was associated with 9.3 fewer GCSE points at 16 - the equivalent of dropping a grade in two subjects.
Two extra hours of screen-time was associated with 18 fewer points - or dropping a grade in four subjects.
The results also suggested extra time spent watching TV had the most detrimental effect on grades.
Pupils who put in an extra hour of homework or spent the time reading, did better in their GCSEs, scoring 23 points more than the average.
But even if pupils spent more time studying, more time spent watching TV or online, still harmed their results, the analysis suggested.
Extra time on moderate to vigorous physical activity had no effect on academic achievement.
"We believe that programmes aimed at reducing screen time could have important benefits for teenagers' exam grades, as well as their health," said Dr Van Sluijs, of the Medical Research Council's Centre for Diet and Activity Research at Cambridge University.
"It is also encouraging that our results show that greater physical activity does not negatively affect exam results.
"As physical activity has many other benefits, efforts to promote physical activity throughout the day should still be a public health priority."
Lead author Dr Kirsten Corder said the measurements taken on the Year 10 pupils represented "a reliable snapshot of participants' usual behaviour".
She added: "So this is roughly equivalent to two grades lower for one subject, one grade lower in two subjects.
"We followed these students over time so we can be relatively confident of our results and we can cautiously infer that TV viewing may lead to lower GCSE results but we certainly can't be certain."
"Further research is needed to confirm this effect conclusively, but parents who are concerned about their child's GCSE grade might consider limiting his or her screen time."
Dr Corder suggested there could be various reasons for the link, including "substitution of television for other healthier behaviours or behaviours better for academic performance, or perhaps some cognitive mechanisms in the brain".
The study is published in the International Journal of Behavioural Nutrition and Physical Activity.