Sixth-formers are to be paid pound;150 a term to act as mentors to obese primary pupils.
More than 300 16 to 18-year-olds are to be recruited as part of a drive to improve the health and well-being of young people in Liverpool.
The "health mates" will target the city's 11-year-olds, who are causing the most concern. Half of all boys and 40 per cent of girls in that age group are overweight and one in 20 is clinically obese. Up to a third are thought not to do any exercise at all.
The scheme, which will begin in September, is intended to reduce the number of overweight or obese children by half within three years, and to encourage all pupils to do a minimum of two hours' exercise a week.
The programme is also aimed at preparing teenagers for the world of work.
The mentors will have to meet performance targets to receive payment.
The city council had considered paying the teenagers in cash, but the idea was scrapped when it emerged that it might affect some families'
entitlement to benefits. Instead, they will receive vouchers offering free or subsidised travel, CDs, books and concert tickets.
The mentors will be expected to give advice on healthy eating and help to run after-school sports clubs and other activities such as dancing and cooking.
The decision to use teenagers to implement the plan was taken by Liverpool council, which believes that children are more likely to listen to, and be influenced by, their peers than adults.
The scheme, part of Liverpool's Children and Young People's Plan, will also offer free admission to swimming pools for all five to 16-year-olds at weekends, while over-13s will be able to use some gyms free of charge.