Why so many students opting for the international baccalaureate?

A broader and more challenging curriculum is winning pupils over

The International Baccalaureate is looking like the most exciting development in sixth-form education in recent years, with advocates arguing that it could revolutionise the curriculum.

If that sounds a little grand for a qualification only taught at about 100 schools, consider that if registration continues at its current rate, that number could almost double by September.

Since October, 23 schools have registered to offer the diploma this year, among them Wellington College and Cheltenham Ladies' College under the Girls' School Association president Vicky Tuck.

In 2006, the International Baccalaureate was given what looked like a standing ovation by Ucas. Under the tariff, a common IB score of 30 gives a candidate 419 Ucas tariff points, against 360 for three As at A-level. A top score is equivalent to more than six As at A-level. It looks like any school aiming high in the league tables has to teach the IB.

The IB schools insist it's not just about results. "It isn't about the Ucas tariff," Tuck says. She acknowledges that universities' respect for the qualification was a factor, but the decider was when she inspected an IB school for the Independent Schools Inspectorate.

"People become evangelists about the IB," she says. "The mission is noble, the internationalism and fostering understanding about different cultures is quite compelling." Thirty Cheltenham ladies will start the diploma in September.

Some IB schools, such as league-toppers Sevenoaks and King's College School, Wimbledon, now teach only the diploma but a school would be brave to make that leap in one stride. Like most schools, Cheltenham will teach A-levels alongside the IB. In fact, Tuck says, one exciting element of the IB is the greater choice sixth-formers are getting, with the diploma as one option, alongside A-levels and pre-Us. "It's indicative of people wanting young people to have exciting learning, to have to stretch," she says.

Tuck says the cost of setting up the diploma, particularly of taking on extra staff, will prevent smaller independent schools from offering all options. Costs run into tens of thousands of pounds, and charges for exam entry and teacher training are high.

But larger boarding schools are well placed, as they already offer many of the extracurricular activities that form the programme's 150 hours of supervised CAS (creativity, action, service) time.

Andrew Trotman, warden of St Edward's, Oxford, says: "It's probably less of an adjustment to the head count on the academic side than at a day school." Forty pupils will start the diploma at St Edward's in September, after a three-year consultation by the school. "It's been properly thought out and planned," Trotman says. "Educational principles, not the examination, are the priority."

As well as the international aspect of the course, many IB supporters talk enthusiastically of its breadth. On the diploma, students select options (specialising in three) from six required subject groups: first and second languages, humanities, sciences, maths and the arts. They also write an extended essay of 4,000 words, take a theory of knowledge course, and do the 150 hours' CAS time.

"The IB offers genuine breadth and a challenge to people at the top end," Trotman says. That breadth, not results, is driving the IB revolution, say advocates.

"I wasn't sure which three or four subjects I wanted to focus on," says Crystal, 18, a student at Godolphin and Latymer School, which introduced the IB in 2005. "With the diploma, I could keep my options open."

Crystal found she could earn credit for extracurricular activities she was doing anyway, like debating, and took the CAS opportunity to take up golf.

But it's the theory of knowledge course, looking at ways of understanding evidence in the arts and sciences, that most excited her. "I've really enjoyed it. You get to discuss interesting issues, and question the ways we know about things. It's quite relaxed, but you get a lot from it academically."