In ten years at one of the toughest schools in the Welsh Valleys, Aberdare Boys School, science teacher Anne Savan had never met a class like it.

She dreaded going into class because no matter how hard she prepared for lessons, the children just did not want to know. "They lacked co-ordination," she said. "They were often frustrated with the tasks set for them and became aggressive. The whole thing was a stressful situation."

Anne Savan is convinced the music therapy has worked

Yet now some of the most difficult children from the special needs group Anne struggled to control five years ago are quietly sitting their GCSEs. She is convinced that this remarkable turnaround owes everything to the music of Mozart.

"I literally went into the garage and picked up a tape of classical music - the only one was Mozart - could have been anything but I picked up this tape - switched on the tape - and hit on the right composer."

Music therapy

Richard Watson asks what is so special about Mozart's compositions?

When Anne Savan first told hardened colleagues about the idea, they thought she had finally cracked.

Fellow teacher, Graham Jenkins said: "She could have knocked me over with a feather when she said she wanted to play Mozart during the lessons. I had a good laugh about it with the head and basically thought it was a load of nonsense."

They may have been sceptical, but the results for the special needs children were dramatic. Anne said: "They roared down the corridor as normal, but when they reached the door they stopped, walked in, didn't say anything to me or each other - sat down and got on with the task in hand."

Class experiments

Some of the most difficult children are now sitting their GCSEs

She had stumbled on something known as the "Mozart effect". Anne found she could observe the calming effect by measuring a drop in the student's blood pressure while they were listening to the music. Her results now form part of her PhD research on the subject.

One theory is that high frequency sound stimulates an area of the brain called the limbic system - which is believed to play a key role in controlling our emotional responses. Certain kinds of music may be particularly effective at prompting the release of chemicals such as endorphins. Endorphins help reduce blood pressure and that reduces the level of adrenaline and steroids in the body.

Anne said: "The amounts of these chemicals reduce the hyperactivity of the pupils and therefore aid their co-ordination, and because the co-ordination improves, they lose the frustration of not being able to do a task - and if you remove the frustration then you remove the aggression and the impossible behaviour."

The link between music and brain function is well known. Experiments in the 1970s and 1980s demonstrated real physiological changes, such as lower blood pressure and pulse rate - the sort of calming influences seen here at Aberdare. But is the fact that it is Mozart so significant - or would Bach, Gorecki or even Val Doonican have the same effect?

The Tomatis Foundation

At the Tomatis Foundation children are treated for autism and dyslexia

At the Tomatis Foundation in the Sussex town of Lewes, children are treated for a range of problems including autism and dyslexia. Specially filtered music is played to stimulate the inner ear, which in turn stimulates the complex listening responses of the brain.

Over the last 50 years researchers have experimented with many composers - but have found that none can match the therapeutic effect of Mozart.

The latest example of the power of the composer's music is shown in Jason Starkey's story. Jason, who has been in trouble with the law for much of his life, has just started work at the foundation after spending four years in prison for armed robbery.

"Inside prison one day someone came up to me and said did I want music therapy," said Jason. "I tried it out. I slept better, stopped reacting to aggressive behaviour and when I walked away I didn't feel I lost face. I felt a lot more in control."

Jason believes that listening to Mozart has given him a new chance in life.

The Mozart puzzle

Paul Robertson: "I'm sceptical about it just being Mozart"

Professional violinist, Paul Robertson, who has conducted research into how the brain is stimulated has an idea about what is behind the Mozart effect:

"It's possible that Mozart had Tourette's Syndrome, involuntary swearing, etc, but he was very ordered when he was playing music. He may have been using his music to organise himself, so when we listen it may be that we are being beautifully organised in some way."

But he believes we simply do not know enough to say that therapeutic effects are limited to Mozart.

"Personally I'm sceptical about it just being Mozart. Scientists... just haven't looked elsewhere," he said.

Before Mozart, few of the children had the patience to sing in a choir

Anne Savan said she tried other composers, but none of them worked. She said that before Mozart, some of the children could not clap in time, and few would have had the patience to sing in a choir which now travels the area.