Pupils at the specialist sports college used to buy 2,000 bags of crisps a week. Now they queue up at the vending machine for some dried fruit before disappearing into the dance studio or up the climbing wall.
"We have a whole-school approach to health, and obesity is an element of that," says Mr Harper.
"We realised we were offering opportunities to the gifted and talented, but were not doing anything for the others whose needs are just as great."
So he got in touch with Leeds Metropolitan university, home to Paul Gately's weight loss management camps.
Now, as part of a compulsory fitness test, all Years 7,8 and 9 students are weighed and measured. "We test their academic abilities, so why not their physical abilities? " "Schools have thrown up their hands in horror and said you can't weigh students, you can't fitness test them, it's an invasion of privacy. But we look at it from the other perspective.
"These kids know they are fat, but the environment they are living in with so many fast-food outlets and so much energy-dense food doesn't help them do anything about it. We're just trying to counterbalance that."
The parents of obese and overweight students identified by the fitness test are invited to an evening meeting at the school. Then, if they choose, they can tell their children they have been offered membership of the Carnegie Club.
About 15 students currently attend the club's three after-school sessions a week run by university staff. Two involve physical activity and one is on lifestyle, aiming to show them that they can do something about their weight and that there are people around to support them.
The atmosphere is relaxed. Students call staff by their first names, encouraging them to chat about issues they might not discuss with a teacher.
Feedback has been positive, says Mr Harper, and interim results are promising. "We try to find out what activities they enjoy and encourage them to continue. If you have an obese student you are honour-bound to try to help."