Stonelaw High received pound;1,300 from South Lanarkshire Council and bought 300 pedometers.
It carried out the month-long project with 180 of its 240 S2 pupils, who had been divided into 60 of the most talented, 60 of middle ability, and 60 who were the least fit.
Some 40 parents took up the school's invitation to take part, too, as did 50 teachers.
Each person had the pedometer set to hisher own stride length and was asked to wear the device all day for as many days as possible over the month.
The aim was to calculate the average number of steps the pupils took each day, plus the distance covered. At the end of the month, approximately 50 per cent of the pupils returned their cards, and the results were analysed on the basis of weekday exercise and weekend exercise.
The results showed the talented section took 11,500 steps on average during the week and 9,000 at the weekend. By contrast, the fitness section ran up 7,000 weekday steps and 6,900 at the weekend.
Mr Byrne said the results confirmed the staff's worst suspicions. "People who are generally unfit do not expend enough energy in a day to maintain a reasonable fitness level, far less improve upon it," he said. "The probability is that, not only do they not take enough exercise, these would be the pupils who also will have a less healthy diet."
The school now plans to test pupils in the future against this year's results, which will help staff to gauge objectively whether fitness levels are improving or not.
Schools should be trying to encourage all their pupils to walk a minimum of 10,000 steps per day, Mr Byrne believes.
Wearing the pedometers motivated the youngsters to improve their scores, the school found.