Russell Martin is in an elite group – one of a handful of teenagers worldwide who learnt last week that he had achieved a perfect score in his International Baccalaureate exams.

It’s an extraordinary achievement, equivalent to getting at least six grade As at A-level. Only 65 (0.17%) of the 38,200 pupils who sat the exam last year scored the perfect 45 points. This year Martin’s school, the fee-paying Sevenoaks in Kent, had nine pupils who obtained the top score. Four are now heading for Oxford or Cambridge and two are going to Ivy League American universities.

“The baccalaureate is a more difficult exam than A-level in that you have to perform well across six subjects – you can’t just specialise in three sciences or three arts subjects as you can at A-level,” says Martin, who switched from his grammar school in Buckinghamshire, to take it.

“When my friends were doing A-level revision they were very relaxed, even revising just the night before. With the baccalaureate you don’t have that luxury. The amount you need to know means there is no way you could get away with a couple of days swotting just before the exam.”

The IB, which is seen as tougher than A-levels, especially for high-flyers, is growing in popularity in state and private schools. Yet even as more schools adopt it, disquiet is mounting among parents and pupils that top British universities – baffled, perhaps, by the scoring system – are asking baccalaureate students to jump unfairly high hurdles. While the standard offer of a place at Oxford and Cambridge depends on obtaining three grade As at A-level, some IB pupils have been asked to score up to 43 points, equivalent to twice as many A-level A grades.

Tommy Gill, 18, a pupil at Sevenoaks, is a case in point. He was astonished when a Cambridge college offered him a place last year – as long as he notched up 43 points in the IB. “I was a bit shocked,” said Gill last week, speaking from Cornwall where he is on holiday. “I was expecting a lower offer, one within the range of 38-42 points which is what the university tends to give. The school wrote to the college but they said they felt justified in making it.”

Happily, Gill was one of the Sevenoaks nine who achieved a perfect score and so he will be starting a four-year degree in medieval and modern languages at Christ’s College. Nonetheless, he said, he was worried both by the level of the offer and the reaction of some Cambridge colleges to the IB.

“At another college one admissions tutor said he knew all about the IB but then asked me how many higher level subjects I was taking. I didn’t apply there because I felt that anyone who didn’t know the basic structure of the IB – that there are three subjects taken at a higher level and another three at a standard level – was not well informed about the exam.”

Kelvin Bowkett, admissions tutor at Christ’s College, defended occasional high IB offers. The official conversion scale, which says 45 IB points is equivalent to at least six A grades at A-level, is “absolute rubbish”, he said. “Normally we would make an offer of around 39 to 40 points. If it were higher there would be a reason for that.”

However, parents writing to Chris Woodhead’s column last week raised similar concerns to Gill’s. While asking to remain anonymous, one said that her daughter had just received her IB results – she had scored 35 points, which she believed was “equivalent to 4½ A grades at A-level”. Yet her first-choice university had asked for 38 points. Her mother said: “I do not understand how they can ask IB students for such high marks when A-level candidates can only be asked for three As.”

Another parent wrote in to complain: “Our experience is that IB students are heavily discriminated against and are given much higher university offers to achieve – sometimes double that of an A-level based offer.”

Martin avoided such pitfalls last year by turning his back on British universities. The 18-year-old says that American universities understand the IB and students have a better chance of a place at Ivy League universities such as Yale.

Although Martin had considered applying to Oxford, when Yale offered him a place, irrespective of what score he got in the IB, he decided to focus his efforts across the Atlantic, eventually plumping for the University of North Carolina, which also gave him a $150,000 (£75,000) scholarship over four years. Last week he went to America to start a leadership mountaineering course, part of his university package.

“The admissions tutor at Yale said that when she saw the IB on the application form it boosted me through the applications process,” he explained, “because Yale is looking for all-rounders who want leadership opportunities. As soon as they see the IB they know you have ticked those boxes.”

Martin is a friend of Gill and says he was surprised by Cambridge’s response: “People who are asked for more than 40 points sometimes complain that Oxford and Cambridge don’t understand the IB.

“I agree that maybe there isn’t always a realisation of just how difficult it is to get more than 40 points compared with getting, say, three grade As at A-level . . . It does sometimes seem unfair.”

Mike Bolton, undermaster at Sevenoaks, says that the “average worldwide IB score is around 30. Those scoring 45 points join a genuine international elite and universities know that they’re getting something special.”

HOW THEY COMPARE

45 IB points = 6.4 A-level A grades

40 IB points = 5.4 A-level A grades

35 IB points = 4.5 A-level A grades

30 IB points = 3.5 A-level A grades

Calculated from Ucas data Research: Tom Coulson

Times Recommends

• The bright side of this winter rain
• Story behind the picture: Snow Leopard, 2008
• Times Charity Appeal: The Place2Be