Patricia Head, sixth-form tutor at Kenilworth School, a large comprehensive in Warwickshire, calculates that pupils should be doing five hours study a week per subject, beyond the taught timetable. That's 15 hours for someone doing three subjects - "they might manage four hours in school study periods, which leaves 10 or 11 to be done at home," she says.
This seems manageable enough - an hour-and-a-half for each of four nights, Friday night off and a big push at the weekend.
But paid work cuts into this time. Ask any group of sixth-formers and almost every one has a job - an impression supported by Warwickshire's inspectors who found, in a survey of the county's 19 sixth forms, more than 70 per cent doing paid work.
Evidence suggests this phenomenon is increasing over time. In 1992, a research study by Peter Tymms and Carol Fitz-Gibbon using data from the Durham University based A-level Information Service (ALIS) reported that 50 per cent of A-level students were in paid employment. Warwickshire inspectors reported that in almost every case, sixth-form tutors, asked to rank their concerns, put part-time work first or second on the list. They're not worried so much about the paid work, but the long hours that many of their students are putting in.
The Warwickshire survey, in reporting this, refers to the Tymms and Fitz-Gibbon research when it says that more than nine hours a week has "a negative effect" on A-level success. But from what sixth-formers told researchers, nearly 30 per cent work longer than this, with 54 (2 per cent) working more than 20 hours in paid employment.
In fact, a close look at the ALIS-based study reveals that its findings on the negative effects are perhaps not as clear-cut as the Warwickshire study survey implies (see right). Nevertheless, common sense and the professional judgment of teachers supports the view that because A-level study is hard, there comes a point where part-time work starts to interfere.
It is easy to see why the teachers are worried. From just two groups of first-year sixth-formers - around 30 in all - I found John, who works almost 25 hours a week in a shop, Janet who works 20 hours on telesales, and the quite remarkable Geoff who, despite being signed up to three A-levels, claims to work 33 hours each week at a local hotel.
Only a handful had chosen not to work - one, for example, because of her concern at her school workload, another because his life is already full as he belongs to a jazz band and a theatre group.
Pat Styles, long-serving head of sixth form at Nicholas Chamberlaine School in Warwickshire, knows that once a pupil has established a reputation as a reliable worker, supervisors start to ask for more hours. This tendency, in her experience, has increased over the years. "That's the real problem. The student does a couple of nights, then the employer starts to push for more shifts, with a hint of 'do you want to keep the job?'" Patricia Head believes that some employers are "getting articulate, able and cheap labour, covering unsocial hours". She agrees with Pat Styles about pressure - "they'll say 'just do it for the next three weeks to help us out'."
There is lots of evidence that employers like the flexibility offered by young people. Some branches of the catering industry seem to survive by having workers waiting at the end of a phone. "They'll phone me up at half an hour's notice and ask me to come in," said one student, and another told of being phoned "at 11.30 at night to go in the next evening."
Their labour is certainly cheap, too. I asked a large group of sixth-formers what would count as, respectively, good and bad rates of pay. They were unanimous - Pounds 4 an hour is good, less than Pounds 3 is bad. Tesco, for example, pays a 17-year-old checkout operator Pounds 3.63 an hour (Pounds 4.40 in London). When I asked the supermarket chain about its policy towards sixth-formers, a spokesperson told me they would "typically" work two evenings and Saturday. "If an employee wishes to work more than this we would ask him or her to consider the implications involved to discourage them from working excessive hours."
At Dr Challoner's Grammar in Buckinghamshire - one of the top achieving boys' schools in the country - around 25 sixth-formers are employed as cleaners earning Pounds 3.75 an hour. But there are strict guidelines limiting the hours worked - a maximum of six a week - with contracts terminated two terms before exams start.
Graham Hill, the headteacher, writes to all sixth-form parents giving advice about how students spend their time, given the need for five hours homework per week on each A-level subject. "We can give advice and make rules for those who are cleaners but, of course, we can't control what they do all day Saturday and Sunday. We think it can be a valuable experience to have a job but taken to extremes, it can be damaging."
Cleaning by pupils is not uncommon in Buckinghamshire schools, partly because of a local labour shortage for low paid jobs. At Dr Challoner's, where Oxbridge candidates have been known to mop, clean and vacuum, boys are given training, supervision and a pay rise to Pounds 4 an hour if their work is excellent. "It's a proper job which is also a service to the school," says Peter Gee, deputy head. "The lads are very keen but if they don't shape up, they lose the work - which we've had to tell someone this morning, actually."
Patricia Head believes that the proportion of sixth-formers working "has nothing to do with the affluence of the students". The more likely motive is the desire to have a degree of independence from the family - and perhaps to pay for entertainment and driving lessons.
So what's wrong with young people learning something about independence and the value of honest toil, especially when pressure to earn is likely to grow in higher education? Certainly most sixth-formers are keenly aware of the advantages of being able to demonstrate to admissions tutors and employers that they have done something productive outside school. Pat Styles, when she surveyed her own upper sixth prior to their leaving, said that the ones who had never worked "perceived themselves at a disadvantage".
Neither do many sixth-formers admit to hating their jobs. One girl was enthusiastic about her work as a care assistant in a retirement home. Tracy Garner, 17, who serves customers in a fish and chop shop on four evenings a week, says she "really enjoys it, the people I work with are really nice. I get to know the regulars and I socialise with everyone." Liz Ellis, 17, with a Saturday job at Woolworths, beamed at the very thought of her work. "The people are nice and kind. We go bowling together and out for meals."
But can a 17-year-old work, say, 20 hours a week as a waiter or chambermaid and still do justice to A-levels? Pat Styles believes that excessive working hours means at least one of the students I met is heading for failure. The tides and pressures that push students to work, though, are difficult to stand against.
Patricia Head says her school counsels pupils and their parents individually - "but it's not easy. We've considered putting something in a home-school contract." Pat Styles has thought of approaching the problem from the other hand - "perhaps a county-level letter to employers."
Some evidence suggests that as the exams approach, students go out to work less often - "if they have two jobs they drop one," says Patricia Head. Nevertheless, she found that more than three-quarters of her upper sixth were still working as late as last April.
It is not always clear by just how much any individual student's studies are affected - teachers know that some really can do long hours and get good results. They also cite cases where students who are already struggling and headed for failure deliberately take on lots of paid work as a form of opting out, an excuse to do badly in exams.
But the Tymms - Fitz-Gibbon study reports that some high-achieving students worked several hours of part-time employment. "The fact that they can do it and still get good grades says something about them," says Pat Styles.