Young children who do not exercise very much are more likely to perform better in exams as they grow up, according to researchers at the Institute of Obesity, Glasgow.
Avoidance of exercise particularly helps girls do better in written exams in P.E., as opposed to practical assessments - "which nobody knows how to mark, anyway," said one fat girl.
The researchers measured the duration and intensity of 5,000 children's physical activity levels when they were aged 11. The children had to wear an accelerometer, worn on an elasticated belt which also indicated waist size.
By the age of 15/16, exam results improved for every two additional hours of non-activity per day by boys, and every one additional hour for girls.
Compared with boys, girls are usually better at sitting still - which helps when sitting exams. Girls are also better at re-sits, and baby-sitting.
"Fit, active kids don't sit still long enough to finish writing," said one local teacher. "Some don't even turn up for exams - they prefer to be outside, running and biking and stuff."
One school is already considering hiring dog trainers to encourage the more active pupils to "sit!".