A study that tracked the progress of 845 students between the ages of 14 and 16 found that every extra hour they devoted to TV viewing, internet use, or playing computer games reduced their scores by the equivalent of two grades.
While reading and homework improved performance, it could not make up for long periods of time in front of a screen.
The scientists stress that what they found was an association, not proof of a causal link.
However, they have ruled out the influence of poverty or affluence, the students, who attended 18 different schools in Cambridgeshire and Suffolk, came from a diverse range of social backgrounds.
Lead researcher Dr Kirsten Corder, from Cambridge University, said: “Television, computer games, and internet use were all harmful to academic performance, but TV viewing was the most detrimental.
“We can cautiously infer that increased screen time may lead to poorer academic performance for GCSEs.
“I certainly wouldn’t recommend banning television. But if teenagers or parents are concerned about GCSE results, one thing might be to look at the amount of TV viewing that they’re doing and maybe just try to be sensible about it.”
The research was part of a large study looking at different factors affecting the mental health, well-being and academic achievement of teenagers.
Between 2005 and 2007, the scientists measured activity levels of participants using heart rate and movement sensors attached to their bodies.
They also questioned the students about how much time they spent in front of TV or computer screens, doing homework, or reading for pleasure.
GCSE performance was assessed at 16, by adding together all the points students obtained across different subjects.
Points are awarded for different grades on a descending scale. An A star, for instance, earns 58 points and an A, 52.
Every extra hour spent watching TV or online was associated with 9.3 fewer GCSE points overall at age 16, equivalent to the difference between two grades, for instance dropping from a B to a D.