But there is another aspect to the stress that is placed on our students - the pressure of holding down a paid job.

In a recent mid-term test for my A-level biology class, I was stunned to see relatively able students performing well below my expectations. The test was not set out of the blue; all year 12 students were aware that there was an exam week looming.

I was even more shocked by the mid-term test results for A-level chemistry taken by the same group of students. Their disappointing underachievement was mirrored in that subject as well as in mine.

Immediate action was needed to find out what was going on and to nail the reasons for this complete failure to perform in the tests. The answer was simple enough: the students had not spent enough time revising. But this was not merely down to laziness.

A survey, albeit limited and impromptu, revealed each student was clocking up an average of 12 hours part-time paid employment per week. After a 25-hour school week, socialising and the odd driving lesson, there simply was not the time left to devote to their A-level studies. It was not so surprising, then, that they got such low marks.

Some serious discussion followed between myself, other members of staff and the students themselves which resulted in retests, in which their performance improved significantly. Whether this effect will be maintained, only time will tell.

The experience set me thinking. I have a relatively small A-level group of nine students, but I wondered what the level of part-time work was among a significantly bigger year-11 group. So I conducted a second survey which revealed some startling statistics. Nearly half the year group (44 per cent) were doing paid, part-time jobs - excluding baby sitting, something they did not consider real work - with students working, on average, 7.5 hours each school week.

Using the results of the science mock exams, sat by all students a few weeks earlier, the number of hours worked per working student was plotted against their position in the year group. Not surprisingly, there was a direct correlation between the time spent working and the student’s position in the exams. The top 20 students averaged 5.5 hours per week, and the next 20 six hours, rising to more than nine hours per week for those with the lowest ranking class positions, between 81 and 100.

A number of issues arise out of these findings. From the point of view of the school, we always insist that homework takes a high priority with both students and parents. Apart from reinforcing the key concepts learned in class, a lot of subjects need homework time in year 11 to ensure coursework -that could play a significant part in a student’s final examination result - is completed.

With students earning on average Pounds 3.50 per hour for part-time work, what incentive do they have to complete homework for their exams? When you combine the time they have spent at work and at school, without including home study, some have already put in a 35-hour working week. And students will often do jobs late at night or at weekends, covering the unsociable hours which full-time adults do not want to do. In effect, quite a few of them are “employed” seven days a week. There must be a knock-on effect in terms of tiredness and motivation.

How discerning are local employers in employing students who are in full-time education and asking them to work extra shifts to cover for absent employees? What will happen when the students are on study leave next term, supposedly preparing for their public examinations? Their employers might persuade students (or the students might themselves ask) to work during this time, when they should be concentrating their efforts on study or their exams.

There are laws relating to the employment of young people. How widely are they being abused? Are all those in part-time employment legally there? It would be foolish to think that the illegal employment of young people can be wiped out, but I suspect it is more widespread than we would comfortably like to admit.

There is no doubt young people can and do benefit from part-time employment. Work experience, for example, is a valuable part of their education. I am sure a lot of parents rightly encourage their children to get part-time jobs to pay for the luxuries some of them take for granted - a constant supply of CDs, the latest trainers and fashion accessories and, for older students, driving lessons and cars. There are also cases of students who need a job to supplement the family income.

One of my concerns, however, is that the thirst some students have for money becomes a Catch-22. Students need money for personal items or to fund a social life. The more they work, the more they can earn, the more they earn, the more they can afford, the more they like the freedom to buy and socialise, the more they have to work.

Another concern involves the desire for local employers to cut back on staff wages by employing young people at well below the going rate for adult workers. On the other hand, some employers take their responsibilities to both employees and customers very seriously, and they will insist that new employees are trained in order to give the right customer satisfaction. In these circumstances, are employers demanding a degree of loyalty from students that conflicts with the commitment they should be giving to their studies?

It may be that my school is not typical but I am sure we are not alone in having to battle with this unseen obstacle to the already tough task of raising standards.

The senior management of any school is now looking, quite rightly, at raising standards, improving the GCSE and other examination results and asking staff to focus on those students who are identified as under-performing or who have a realistic chance of increasing their grades with help and the correct educational focus. League tables are a big issue.

But are we in competition with employers? The group of students who should move from that grey area of GCSE grade D, to the magic C are, by and large, the group who, in my analysis, are working the highest number of hours per week.

How can teachers get the best from students who, when offered after-school revision classes to help boost their grades, say they’re sorry they can’t come because they’ve got a job to go to? A voluntary revision class is one thing, but I am left speechless when the excuse for not doing a well-deserved detention is: “Sorry sir, I can’t. I’ve got me job to think of, I might get the sack if I don’t turn up!”

Schools must look at this problem and discover the scale of part-time work among their pupils. They must then take steps to advise both students and parents of the possible consequences and the legal position of regularly working long hours in addition to having a full school timetable.

As a head of faculty, I have a responsibility to ensure that all students achieve their full potential, or as near to it as possible.The worrying thing is that I have little or no control over who is working, when or where, and how that is going to affect their examination performance.

James Williams is head of the science faculty at The Beacon School, Banstead, Surrey