A controversial US teaching method, applied behavioural analysis, or ABA, is now being used in a few UK state schools. And parents are moving from all parts of the country to get their child a place

A year ago, Julie Barber came close to a nervous breakdown. Her son, Jack, who is four, has autism, and she felt overwhelmed by his demanding behaviour. "He was having two-hour meltdowns nearly every day," says Barber, who lives with Jack in Thurrock, Essex. "I've got a bad back just from trying to manage him." Her greatest worry was that, since the age of three, he had regularly refused to eat anything except baby food and custard, and it had to be a particular brown colour. "It was horrendous – if I tried anything else he would be sick," she says. "Even if a drink was too cold, it would make him gag. I was scared that he'd deteriorate because he wouldn't get the right nutrients."

Today, watching Jack happily tuck into sausages and baked beans, she still finds it difficult to believe how far he's come. "He eats pretty much everything now. I can take him out to a restaurant, or a party. You have no idea what that means."

In September last year, Jack started at Treetops, one of a handful of state special schools in the UK offering a programme of applied behavioural analysis, or ABA.

First developed in California in the 1960s, ABA uses a system of rewards to change children's behaviour and teach them new skills. It has always been controversial – psychologist Ole Ivar Lovaas, who pioneered the use of ABA on children with autism, used "aversives", such as striking children or giving them a mild electric shock, when they did not comply. These punishments have long been abandoned, but critics still warn the method is overly demanding – some programmes involve 40 hours a week of contact time – and have likened the approach to "dog training".

However, ABA has seen a huge rise in popularity and is now widely used in the US as an approach for autism. Online, there is an abundance of ABA success stories, with consultants describing it as a highly effective way to "normalise" children with autism and help them communicate and function in the world.

Demand for ABA is just as high in the UK, but the majority of programmes are in the private sector, meaning schools like Treetops have become very oversubscribed.

"We are at absolute capacity," says the headteacher, Paul Smith, as he strides through the serene, spotless corridors at Treetops, decorated with haiku poems and summer holiday reports written by the children. "Families are moving from all over the country to get their child in. They are desperate. We know that the earlier a child starts on the programme, the better their outcome will be, but, unfortunately, we're getting log jams, which means families are left waiting."

ABA can be used for anything from improving behaviour to teaching curriculum subjects. The key tactic is to engage the child using individual rewards or "reinforcers".

At Treetops, which uses a type of ABA known as verbal behaviour or VB, each child is "paired" with an assistant who carries a bag of "rewards" – toys or props the child enjoys using. Whenever they perform a task correctly, or behave as they are being taught to, they get a few minutes with their reward. In one of the older year groups, a teenage boy is treated to five minutes on the Nintendo DS, while another runs a wooden toy up and down his arm. In the nursery, a teaching assistant simply blows bubbles around the room as a reward for her pupil correctly saying his numbers.

"It works because it's so individualised," says Jennifer Hubbard, an ABA teacher and manager of the school's VB programme. "Each child's programme looks very different and we make it specific to what's going on in their life.

"We tend to start by teaching them how to request things because a lot of problematic behaviour stems from frustration at not being able to communicate, and once they have a way to tell people what they want, or don't want, problem behaviour dec reases."

Many of the pupils at Treetops' ABA centre arrive unable to speak – one boy started aged 13, unable to communicate a single word, say staff. Although some develop full speech, the classrooms are filled with children using sign language or even typing words out on tablets to communicate with their assistant.

Unusually for a school specialising in autism, the classrooms are brightly coloured, with the children's pictures and projects dangling from the ceilings and walls."Some people think our classrooms are too colourful, but we want our children to be able to function in a busy environment because that is what life is like," says Hubbard. "The real world isn't a blank canvas."

Although no one uses the word "punishment", there are "consequences" for bad behaviour. These could be the denial of access to a reward or an activity a child does not enjoy. In Jack's case, to tackle his issues with eating, his teaching assistant would give him a tiny spoonful of regular food, and if he ate it he'd immediately get a spoonful of the baby food he liked, as a reward. Slowly, he stopped gagging so much, but his mother admits she came close to stopping the programme.

"I found it upsetting to see him crying and being sick," she says. "And if he was sick, he wasn't allowed to have his custard afterwards. I found it very harsh and questioned whether it was going to work." But she persisted. "Something told me to keep going with it, and after about six months he was starting to eat."

Jeremiah Cherian, five, also started at Treetops last year. His teachers have used ABA to toilet train him. His behaviour and ability to make eye contact have improved, and he has started communicating with signs. "I can't even think about what would have happened if Jeremiah wasn't here," says his father. "It was such a battle to get into this school – we were on the waiting list for a year and a half. There was no way we could have afforded an independent school."

Smith first learned of ABA when a pupil who was undergoing a private ABA programme at home started at Treetops. After researching the merits of the technique, he asked Thurrock council if they would fund the school to start offering it.

"The local authority were already on board because they were funding home ABA programmes and it would be cheaper to offer it through schools," he says. Treetops now has 85 children on one-to-one ABA programmes.

Smith is convinced that ABA is an effective approach for autism, a condition thought to affect one in 100 children.

"Before we started getting log jams, roughly a third of our children were developing sufficient language and behavioural skills to go back to mainstream school, and another third were able to move to our moderate learning difficulties wing.

"We have taken children from other special schools that have been unable to cope with them, and they have progressed here."

However, he is always realistic with parents, some of whom are hoping for a "cure". "There's no doubt that all the children improve – and some are transformed – but, of course, some don't improve as much."

Smith is now keen for other local authorities to start offering ABA in schools.

But the approach is not without its critics. Perhaps of biggest concern is the principle of changing or removing autistic behaviour. "This is a really contested issue in the autism community," says Dr Liz Pellicano, head of the Centre for Research in Autism and Education at the Institute of Education, University of London.

"Some people hate their autism because it prevents them from doing the things other people do. But others celebrate it, and feel it offers not only challenges but also opportunities.

"Although therapists wouldn't say that they're trying to normalise children with autism, that is the underlying ideology of ABA – to make them indistinguishable from their peers."

This is not only ethically questionable, she says, it could be harmful, too. "Being told there's something wrong with you is going to potentially make you more anxious and more depressed, which is already highly prevalent in people with autism."

Dr Luke Beardon, senior lecturer in autism as Sheffield Hallam University, argues that ABA's focus on behaviour and rewards means that children may not learn how to make decisions for themselves. "Let's say we used ABA to teach a child about inappropriate touching or taking off clothes. That's fine, until they are a young adult and have a girlfriend and suddenly they find they want to do this inappropriate touching. The concept of 'why' is so important."

At Treetops, Hubbard insists that the children learn to transfer their skills. "This isn't just robotic learning. They first learn to say 'apple' to get a reward, but then we find they start to use it socially – they say it when they see it at the supermarket simply because they want to tell you – they want to have a conversation."

She adds that her staff would be "horrified" at the thought that ABA aims to take away a child's autism. "We know they will have autism as a lifelong diagnosis. The only behaviour we are stopping is that which is causing them harm or stopping their learning."

So why isn't ABA being taken up by more state schools? Much of the evidence suggests that autistic children do best with a combination of approaches.

"For a certain percentage of children, ABA is helpful," says Beardon. "But it's best if other approaches are bolted on. There's no one-size-fits-all because every child is different. Somewhat ironically, for people with autism, flexibility is the key."

A documentary about ABA, Autism: Challenging Behaviour, will be shown on BBC4 at 9pm on Tuesday 5 November