Nearly one in 20 are working more than the recommended "safe" limit of 10 hours a week, a survey of 550 young people revealed. More than half of the young people with jobs had come under pressure to extend their working hours.
Last week a fish and chip shop owner in the north of England was fined when a 12-year-old boy was discovered peeling potatoes for wages that were simply portions of fish and chips.
The research will show that the problem is even more severe among sixth- formers, with more than one in 10 putting in over 20 hours a week.
Researchers warned that while some part-time work helped to raise standards, too much time spent on out-of-school jobs was seriously damaging to exam performance.
Head teachers and college principals said the survey was alarming, and warned that school-children and sixth-formers were being used to prop up Britain's increasing culture of open-all-hours trading.
They called for Government guidelines to limit the number of hours being worked outside school.
John Dunford, general secretary of the Secondary Heads Association, said: "Heads are very worried abut this. School-children are cheap, and the demand for work is high. The problem is that it is difficult to control and difficult to monitor. Employers are putting pressure on children to put more hours in. They exert moral leverage because there are always other people wanting work."
The survey, by the Government-funded Further Education Development Agency, found that two-thirds of 16- to 19-year-olds were working more than 10 hours a week.
Researchers found that out-of-school working had little effect on drop- out rates, but warned that "tiredness and stress are commonplace". They claimed the combination of lessons, homework and part-time jobs could make some students exceed the 48-hour maximum working week under the European working-time directive.
Their report said: "Whilst engagement in work for a limited number of hours each week does not appear to have any great negative impact, and in some cases appears to enhance academic performance, beyond this level there is a strong negative correlation between hours worked and examination grades."
Almost half of all those polled felt that young people benefited from having a job. But the same proportion of teenagers said they would prefer not to combine work and study. Only one in 10 said a pounds 40 a week Government grant, being piloted from this month, would make them give up work.
Nick Brown, principal of Oldham Sixth Form College and vice-chairman of the Association of Colleges, said he hoped that Government plans to introduce a means-tested grant for sixth-form students would ease the pressure on the most deprived.
He said: "We take a very hard line on this and say that people have to be prepared to put in the necessary level of work. But it is a problem and I have to tell youngsters that it will affect their studies.
"There are two categories of student and we do have some who have to support themselves and their families. They have to work and it is tough on them.
"But we do live in an increasingly consumer society and most students have jobs in order to pay for their lifestyles."Reuse content