Many parents will have barely raised an eyebrow at Prince Harry’s alleged assertion that he wrote only a "tiny, tiny bit" of his art coursework. "Helping" their children with coursework is something many parents take for granted. Look, their argument goes, I don’t want to cheat, but if everyone else has spent weeks on a project, then my son’s mark will be dragged down if I don’t do the same.

With up to 40 per cent of the final grade resting on coursework in some subjects at GCSE (30 per cent for A-level), the pressure is on parents and teachers to perform miracles. In Harry’s case, it was his teacher, Sarah Forsyth, who claimed she was ordered to compile part of his written coursework (although the exam board concluded that a tape she secretly recorded was "insufficient evidence" to open an investigation).

The trouble with coursework is that it is open to abuse all along the way. It is often written at home, allowing for plenty of "guidance" from parents or internet search engines. The teacher marks the first draft, pointing out the bits that could be improved. Then it’s back home for the second draft and more poring over by anxious parents.

Clearly, children with access to books and computers are going to be at an advantage. And it’s middle-class parents who most manipulate the system, according to Francis Gilbert, the head of English at a state secondary in London and the author of I’m a Teacher, Get Me Out of Here! (Short Books, £9.99).

"I see coursework where I think: ‘How on earth did you produce that, knowing what you produce in class?’ The child swears blind it’s his own work, but it’s clear they have had considerable help at home. It is endemic now, but if you confront one pupil about the ‘help’ they have received, where do you stop?"

A growing number of teachers are questioning the validity of coursework. In a survey for the National Union of Teachers last year, 70 per cent supported it at GCSE level, but expressed "considerable concerns" about help from parents and the internet (where you can buy a tailor-made essay for less than £50).

"You feel like saying to the child: ‘You get an E, but your dad gets an A,’ " says John Dunford, the general secretary of the Secondary Heads Association. "It can be difficult for schools because they want parents to show an interest in coursework. But there’s a fine line between help and cheating."

Where that fine line is drawn is down to the examination boards. Yet, surprisingly, there are no published guidelines for parents, other than the basic rule that they cannot actually write their children’s work. "The moment you put your hand on that piece of work, the rules have been broken," says Stevie Pattison-Dick of Edexcel.

So, what counts as cheating? Taking your child to the Louvre for art history coursework? "No, that’s educational," says Pattison-Dick. What if you see a spelling mistake in the finished work and point it out? Not as long as they look up the spelling themselves. Anyway, pupils are not marked down for bad spelling in coursework. How about downloading a wad of information on Amazonian rainforests at work and emailing it to your child? "That’s OK, because it’s the same as buying them books, but if you tell them where to cut and paste, then that’s wrong."

Bene’t Steinberg, the head of public affairs at OCR, the other large examination board, concurs: "Where your assistance goes from indicating to correcting areas of concern, you have stepped over the line."

He believes parents are facing the same dilemma teachers have had for years. "To what extent do you chivvy them, push them and pull them towards the answer?" he says. "To give them the answer is not educative. In our house, it’s a case of: ‘There’s the encyclopaedia, look it up.’ Where you are stopping your child learning, you are no longer helping. You could help them get better marks, but when they hit university, they are going to crash and burn."

Francis Gilbert was so worried about the abuse of coursework that he switched to a syllabus that offers exams instead. "I felt this was the only way to eradicate cheating – part of me doesn’t care if the department gets rotten results," he says. "But everyone at the school has been very supportive."