Can a prep class prepare you for the terrifying entrance test?

I'm sitting at the back of a small classroom in the London centre for Kaplan, hoping my incipient flu will be an excuse for lacklustre participation. The US exam prep company is a little piece of America nestled alongside the National Portrait Galley. It's all bustling fresh-faced women and computer terminals. I'm here to sit the introductory class with a small group of eager beavers who want to improve their lives by going to business school and are resigned to taking the Graduate Management Admission Test.

If the GMAT is not terrifying enough, Amanda Russo is. She's a bright, energetic woman from Boston, and the class tutor. Her you-can-be-a-winner enthusiasm makes me avoid eye contact so I am not picked on to answer questions. The small group of students, who include a civil engineer, a biochemist and an Australian stockbroker, are made of sterner stuff. Friendly, in an earnest way, they square up to challenge the mighty GMAT computer.

They know that many, though not all, schools require you to take the GMAT. It is almost impossible to get in to the most prestigious US and European institutions without a very good score. But the test is hardly everyone's idea of fun: its impersonal, distinctly American in its style, and hard, consisting of analytical writing, maths (quantitative) and problem solving.

Unlike most European exams, it does not test what you know or how you work out the answer. Still less does it test what kind of person you are, which is perhaps odd when so many business schools place such great emphasis on leadership and the importance of "interpersonal skills".

The GMAT is a computer-adaptive test, which in plain English means that the better you do, the harder the questions become. It is also relentless. The quantitative section throws up 37 questions in 75 minutes. You have to be fast as well as accurate. "Someone coming from a bank background may be able to solve a complex maths problem, but if they can't do it in under two minutes they won't do well on the GMAT," says Heather Mancini, Kaplan's marketing manager for business programmes in London.

The maximum score is 800. That is intergalactic genius level. The average score is about 540, but you are unlikely to become a captain of industry with so measly a result. It is sobering to realise that you need about 680 to win a place at one of the best schools and that the average GMAT score for entrants to Stanford, which, according to Kaplan has the highest results, is 726. I quail at the thought and resolve not to suffer the humiliation of taking the test experimentally to reveal my "baseline score".

My classmates have no choice. They have paid £1,045 for a four-week Kaplan course that promises them their money back or a repeat if they do not improve their baseline scores when the big test day comes. For their money, they get hefty course books, online support, test exams, and the exhortatory Russo for nine sessions of two-and-a-half hours each, put on twice a week. But what they really get is plenty of practice and handy tricks.

"GMAT is not a test you can cram for. You need practice, practice, practice. Most students have never taken a computer-adaptive test," says Steven Helgeson, director of the London centre of Kaplan.

I am impressed by the tricks. After a couple of baffling algebra questions, I admire the simple dodge: pick a number. If you substitute sensible looking numbers for letters or symbols, the task is instantly a lot easier. For the English language part of the test, Kaplan has a whole set of techniques it calls "strategic reading", whose tips include how to spot keywords in a passage.

Daunting as it is, there are plenty of willing victims for the GMAT. According to the Graduate Management Admission Council, which administers the test, the worldwide number of people taking the GMAT has risen by a third over five years to a record 265,613 in the test year.

The number of non-US citizens taking the test last year was more than half the total for the first time. European candidates increased by 30 per cent over the five years to 23,224, and significantly more of them are sending their score reports to European, rather than US, business schools. In the UK, 4,900 people took the GMAT in 2008-9, which is almost a quarter of all the candidates in Europe and enough to place the country in the top six worldwide. All of which is good business for the GMAT testing industry.