A 16-year-old pedals vigorously on an exercise bike. Next to her, a younger girl is being put through her paces on a treadmill, while in the corner, a boy lifts small piles of weights.
These youngsters are taking part in a new health scheme designed to combat obesity. Westminster council has prescribed a six-month dose of exercise.
The pupils, aged between 11 and 16, are referred by their family doctors.
They are then offered 12 weeks' free membership of a local gym, supplemented by a further three months' membership for pound;1 a session.
Eugene Minogue, who runs the scheme, said: "We're taking children who wouldn't normally exercise or go to the gym. We're educating them. A lot of children come from areas of social deprivation, and say they can't afford to exercise. We're removing barriers to participation."
The children are given a health screening, in which their blood pressure and body-mass index are measured. Mr Minogue then supervises weekly sessions, in which each pupil is given a personal exercise programme. This incorporates equipment such as exercise bikes, rowing machines and treadmills. The pupils and their parents are given healthy-eating guidance.
"There's no point doing all that exercise if they're not getting a balanced diet at home," said Mr Minogue. "We need parents to come along.
Eleven-year-olds don't cook their own meals. They don't pack their own lunches."
After six months, pupils are encouraged to exercise outside the gym. Andy Durrant, Westminster's school sports co-ordinator, said: "We want to attract the people who don't bring PE kits into school, who get picked on in the changing-rooms, who don't realise PE is something they can do."
Barbara Mayne, head of Queen's Park primary, has referred four pupils on to the scheme. She said: "Many families don't want their kids playing outside on the city streets. This shows them that there is an alternative to flopping down in front of the telly or PlayStation."