Retailers are putting pressure on sixth-formers to work long hours - which is adversely affecting their A-level performance, it has been claimed.
The heads of two Surrey schools are so concerned about the performance of their "exhausted" sixth-formers they are sending out advisory letters to employers who ask for references.
The letters say they do not want students accepting employment during the school day or taking on more than eight hours' work a week.
Their action comes in the wake of moves by Chris Pond, the new Labour MP for Gravesham, Kent, whose private member's Bill is to go before Parliament this month calling for a cut in the number of hours children can work.
Margaret Edwards, head of Wallington High School, Sutton, has also written to local MP Thomas Brake, saying students feel under pressure to work all the hours they are asked to because of threats of the sack, promises of a chance to transfer to a store near their future university or parental worries about the cost of university education.
"We believe students studying full-time can cope with a maximum of eight hours' paid work each week. Unfortunately, our students are experiencing enormous pressure from the managers of stores to work up to 30 hours each week," she said.
The school estimates that 87 per cent of its sixth-formers are working, with at least one in five doing more than 10 hours a week.
While some are working to pay for cars or gym membership, others with less well-off parents say they need the money to pay for school books, field trips and to save towards university.
One sixth-former sat an A-level exam last summer wearing her supermarket uniform to make sure she was back at work in time for her shift, said Cathy Allison, Wallington high's head of English.
"If you look at patterns of student employment, as you go to the tail end of the results, there are your full-time supermarket workers. "
Chris Tarrant, head of Wilson's , a boys' school also in Wallington, believes more students are working longer - and often for social as much as financial reasons.
"One wants to encourage the pupils to work. It is good to manage your own finances and it gives you a bit more freedom if you can run your own car," he said.
"But it is an issue that potentially is going to affect our results. The thing is to know the boys are fulfilling their potential at this stage of their lives."
Both schools would like to see employers agree not to employ full-time students for more than eight hours a week.
Amanda Dance, a spokeswoman for the Waitrose supermarket chain, who has worked in about 10 stores and has been involved in recruiting weekend staff, said she had sometimes had to discourage students from taking on more hours.
"It has to be very carefully handled during term-time and that is very much the message from head office," she said. "Their education is the most important thing - we are just a fill-in, although sometimes we can lead to a career."
She said the company recruited good part-timers to its management trainee scheme, and arranged transfers to stores near universities for capable staff, at the student's request.
But she rejected claims that students might be asked to help out with extra hours in return for guarantees of future work when at university.
Working hours were discussed at interview, with contracts generally stating an average of around 10-11 hours, with the stipulation that employees may be required at busy times to work additional or alternative hours - subject to two weeks' notice where possible.
Derek Hewett, a secondary inspector in Sutton, said that the issue had been raised with him by several Surrey heads and was being looked at by the education authority.
Union research suggest that under-18s are working long hours even for reputable firms. Rory Faulkener, chairperson of the GMB union's young members advisory committee, said: "Employers using schoolkids are saving themselves #163;400 million a year by employing children rather than adults. They might only be paying the sixth-formers in pocket money, but it's pretty sizeable pocket money to the firms."
Peter Rees-Farrell, of the shopworkers' union USDAW, said the problem had come to the union's attention mostly in respect of undergraduate university students.
Sixteen and 17-year-olds had traditionally been paid less than older staff, he said. "Students could be putting their long-term educational prospects in danger for the sake of a quick buck, " he said.