Jane Wainwright, head of Aylesbury High School, Buckinghamshire, has watched increasing numbers of her pupils taking jobs, including some who work for the contractors who clean the school, an LEA-controlled girls' grammar.
The pupils are not motivated by poverty, she says, although some are saving up to get themselves through university.
"I suspect that what I know about is only the tip of the iceberg.
"The younger pupils are supposed to get our permission before they take a job but I know that the forms I receive are much fewer than the children working.
"I will not give permission for children to work below the age of 14 and am very careful even after that. It definitely has a damaging effect on their school work.
"I also have considerable worries about sixth-formers taking employment. In fact I strongly discourage it. They are working more than 30 hours a week on A-levels in school plus the work they have to do at home. What they lose if they take a job as well is all the social and extra-curricular things they ought to be getting out of their time in the sixth form.
"We have done quite a lot of work on A-level failure and it does seem to correlate with students who have taken jobs."