Nearly 50 per cent of 16 to 19-year-olds spend substantial time in employment while many spend as little as 10 hours a week on their studies, say Ann Hodgson and Ken Spours of the Institute of Education in today's TES.
Weaker students are hit hardest by excessive part-time work, say the researchers, who are conducting a review of post-16 education.
Meanwhile, middle-class sixth- formers spend too much potential study time working to maintain extravagant social lives, despite their parents' financial support.
Dr Spours said: "While middle-class parents discourage their children from taking on too much, those new to post-16 education are underperforming because of lack of time to study.
"Weaker students tend to sign up for fewer A-levels or equivalent courses. They see their studies as being a part-time commitment and are the ones who are more attracted to the labour market.
"We are no longer talking about a simple Saturday job. Many of the big employers will not take a student on unless they work a minimum of 18 hours - which is more than many spend on their studies."
While significant numbers of further education students took jobs to avoid poverty, sixth-formers often spent earnings on extravagant social lives, said Dr Spours.
The researchers warn that the phenomenon has affected "schools' appetite for curriculum reform" claiming they are afraid to push for a broader curriculum or insist on more commitment from students for fear of losing them to less-demanding rivals.
The minimum wage, with a lower rate for young people, will see the situation spiral out of control because students will become even more attractive to employers than adults, said Dr Spours.
The warning comes a week after research commissioned by the Department for Education and Employment revealed that more than half of further education students suffered financial hardship.
Earlier this year, a research project at a West Midlands college showed that sixth-formers who spent more than five hours a week in jobs did less well than expected. Students who work for more than 15 hours were the worst affected, dropping more than eight A-level points - equivalent to falling from expected BBB grades to CCD.