The IB Organisation, based in Geneva, says the diploma is designed "for highly motivated students who hope to attend university". Likewise the Qualifications and Curriculum Authority says the IB is for those who are "highly committed and internationally minded". But an increasing number of independent (and state) schools have found that the IB is equally suited to less academically gifted students.
"We have a comprehensive intake and have had great success with what you might call middle-of-the-roaders," says Peter Armstrong, head of Worth School in Sussex. "We are not a highly academic selective school, we have a range of students. Some are very bright, some are average."
There's no doubt that the curriculum load of the IB is far higher than A-levels. IB students choose six subjects, rather than three. But Armstrong says that, while the IB demands more hours in the classroom than A-levels, the structure of the qualification can actually help some students and for those who are disorganised "it's been the saving of them".
The fact that three of the subjects can be taken at standard level means the IB is also accessible. If a pupil's strength is in maths, they study that subject at higher level, and if their weakness is in English then they take that at standard level.
For some students it is the way the IB is assessed that is the big draw, with both written and oral exams geared towards how well pupils can communicate. In English, for example, there is an oral assessment. And for those with learning problems, or who are simply scared stiff by exams, there are subjects such as theatre arts that have no formal written exam at all.
In Worth's most recent IB results, students gained an average of 33 points out of a possible 45, and were made "fairly generous" offers from universities. Nick Connolly, the school's IB coordinator, says because the course is more holistic than A-levels, students have more to write when they come to fill in section 10 of their UCAS forms: the personal statement.
Another bonus is that IB students are rewarded for their cultural, athletic and community activities, by completing 150 hours' community service. This could mean working with elderly people in their borough or working abroad with street children in Tanzania. The aim of the IB is to provide an education based on intercultural understanding (helpful for those thinking of living abroad). This means, for example, that there is a vast range of texts that can be studied for the English literature exam.
Similarly, the IB history syllabus is built around modern world history, whereas A-level is more tightly focused - with a compulsory British history element.
UK schools offering the IB vary, from selective private schools to inner-city comprehensives. Southbank International School in London dropped A-levels 21 years ago. "We didn't pick the brilliant people to do IB, everyone did it," says Nigel Hughes, the head. "You need to be highly motivated, yes. But you don't need to be highly brilliant."
He cites one state school where pupils with an average D grade in GCSE go on to take the IB in the sixth from. They might not all achieve the diploma, but many go on to further education. While the IB is seen as attractive to able students, its fans believe it makes the most of any pupil's potential.