Sixth-formers whose part-time jobs take up more than five hours a week are risking the A-level grades they need to make it to university.
A small research project at a West Midlands college shows that teenagers working up to five hours a week actually improve marginally on their predicted A-level performance.
But results start to suffer when working hours rise above five a week, while those putting in more than 15 hours are scoring an average of four grades below their potential.
The research, carried out by lecturer Ian Howard, programme manager for biology at Halesowen College in the West Midlands, covers a small sample of 88 students who sat their exams last summer.
Potential A-level performance was based on actual grades achieved at GCSE, using a value-added scheme devised by Greenhead College, Huddersfield.
Those working up to five hours scored above predictions by just over two A-level points (where an A grade equals ten points, and an E grade two). Students working between five and 15 hours underperformed by nearly three points.
The biggest effect was among students putting in more than 15 hours. Their performance was more than eight points below predictions - the equivalent of dropping from an expected "BBB" to "CCD," a difference that could well affect entry to university courses.
However, an American study of 12,335 high-school students found no compelling evidence that employment affects grades.
Researchers at the University of Wisconsin revealed earlier this year that it was school performance that determined the numbers of hours students could work, rather than the other way round.
Mr Howard, while acknowledging the small sample and the limitations of value-added schemes, said the study was a worthwhile way of extending the debate on factors affecting student achievement."This data could be used with A-level students to raise discussion about time spent in work and in study. Perhaps it may help them to make informed decisions about their own work and study patterns."
He added: "A little bit of work is a good thing but too much is not. If you have a small part-time job, you have to learn things like communication, reliability, how to mix with other people - and that improves your overall ability and increases self-esteem."
Earlier this year, two Surrey schools started writing to local employers about their concerns that students were being asked to work long hours - adversely affecting their studies.
When companies request school references for new employees, Wallington High School and Wilson's School in Sutton now include a letter recommending a maximum working week of eight hours.
Some retail firms claim to be concerned about students' education and sometimes have to discourage students from taking on too many hours.
But one retail manager, who did not want to be identified, said some stores were inflexible about giving time off to sixth-formers in the middle of revision and exams.
She claimed that in some cases, students are pressed to do more hours than they want to and have been forced to give up their part-time jobs altogether.
* A 15-year-old schoolgirl who had a weekend job operating heart monitoring machines at a Scottish hospital has been dismissed.
Perth and Kinross NHS Trust initially defended the appointment, but subsequently acknowledged there were sensitivities about someone so young being involved directly in patient care.