She climbed trees and played in the grounds of the Suffolk boarding school, the last and most famous example of the free school movement of the 1920s. Lessons at Summerhill are optional and extra-curricular activities famously include nude bathing.
Rebelling against the strict schooling in her home city of Tokyo, she even refused to have a bath for the first three weeks.
Today Ms Otsuka, 27, is a photographer whose work has won awards and hangs in London galleries.
Ms Otsuka left Summerhill with three GCSEs, in English, which she learned as a foreign language, photography and chemistry.
When she left, she enrolled at a London sixth-form college, took A-levels and studied photography at Westminster University and the Royal College of Art.
She calls the liberal regime pioneered by A S Neill at Summerhill "one of the most important experiences of my life".
She said: "Going to Summerhill changed me. I was at a very, very strict Japanese school. The only thing I dreamed was to get out of there.
"When I heard I could go to Summerhill I cried with joy, even though I was only 10."
Her Majesty's Inspectors, who reported last week, told a different story. They branded the school's curriculum "fragmented, disjointed, narrow and likely adversely to affect pupil's future options".
The school was failing its pupils, they said. Children were failing to reach their potential.
Their report said: "This amounts to an abrogation of educational responsibility and a failure of management and leadership. The school has drifted into confusing educational freedom with the negative right not to be taught. As a result many pupils have been allowed to mistake the pursuit of idleness for the exercise of personal liberty."
David Blunkett, the Secretary of State for Education, is preparing to issue a formal notice of complaint, giving the school six months to improve. Officials say their action is not an attack on progressive education, merely a demand that children's "minimum entitlement" to education is met.
Ms Otsuka, however, did not see it the inspectors' way. "At the beginning I did not know what to do with the freedom," she said. "I did all the things I wanted to do. I refused to have a bath for the first three weeks and I didn't change my clothes.
"For the first two years I didn't do anything. Your main enemy becomes the boredom and the only option for me was lessons.
"But it was my conscious decision and that was why Summerhill really worked for me. There was nothing hard about it.
"It's so important to have a school like that, to have a choice."
Dominic Rannie arrived at the school aged 13, barely knowing how to write. He was brought up in a community on a smallholding in rural Herefordshire and had no experience of formal education.
He left with five GCSEs and is now a research assistant at Edinburgh University working on molecular biology and planning to start PhD studies next year.
"To me it was absolutely crucial," he said. "If I had been forced to get an education straight away I would have found it quite a shock. If I had not been there I would not have been so interested in education. I certainly would not have a degree and [be] thinking about a PhD, and I'm not unique."
The school's principal, Zoe Readhead, daughter of the founder, has threatened to take Summerhill abroad if the ethos it has built up over its 78-year history is threatened. She is currently drawing up a list of past pupils' achievements to chronicle the school's successes and prove that its pupils are not sold short.
It is the latest round in a running battle with inspectors which dates back to 1990.
Summerhill's philosophy flies in the face of the rigorous pursuit of targets for raising standards in the basics which is at the heart of government policy.
But it is a philosophy which wins favour with the growing number of people who choose to educate their children at home. And it has forged a strong network of supporters.
The actress Rebecca De Mornay, a former Summerhill pupil, said it would be "a crime" to close her old school. "I learned a whole lot about self- reliance and self-initiative, confidence, imagination and independence, as well as learning much about how society works and how to find one's place in it. Summerhill was an essential tool in my early academic development.
"After two years, I went on to three other 'normal' schools [in different countries] and became a top student in all of them."
Eleven young people left the boarding school in Suffolk last year. One is studying A-levels in maths, fine art and graphic art; another is doing maths, physics and German; while a third is studying biology, maths and physics.
And that, according to Mrs Readhead, is fine. Some get eight top-rated GCSEs and some do not. A questionnaire has been returned by more than 70 former pupils, and she recites their list of academic qualifications and other distinctions and achievements.
The figures show that 20 pupils took GCSEs last year. Of the 69 entries, three quarters passed with grade A*, A, B or C.
"The huge majority of people go on to further education and step into college," Mrs Readhead said. "I find the position of an inspector saying the children are not fulfilling their potential to be outrageous. "A lot of people here are creative and independent ... We are producing ideal citizens from the point of view of the Government. They are well balanced individuals and they feel responsible for their own lives."
LEADING ARTICLE, PAGE 26
SCHOOLING WITH A DIFFERENCE
SUMMERHILL MAY be threatened with closure but parents looking for an "alternative education" still have plenty of choice. At least they do if they can afford to pay. The demands of the national curriculum are such that state schools have little room for manoeuvre. That said, every school is different, some much more liberal than others. So shop around.
If that doesn't prove satisfactory then it is possible to educate children at home.
Up to 10,000 parents are estimated to have taken up this option. It is not necessary to follow the national curriculum but an " adequate education" must be provided. Education Otherwise, the organisation set up to support home education, claims up to 300 parents a month signing up.
Of the "alternative" fee paying schools the following are among the best known:
Nearly 400 pupils, paying pounds 15,000 per year.
Founded in 1893 by a Fabian socialist, J H Badley, who was committed to sexual equality and cold baths, earth closets and manual labour. No uniform and pupils call teachers by their first names. Princess Margaret sent her children here.
Boys only, with fees up to pounds 11,160.
Includes at least 60 acres of parkland, a working dairy, beef, pig, sheep and chicken farm and a fruit and vegetable garden. All pupils are fully involved in both and are required to help with milking at 6.30am each morning.
The Prince of Wales loathed it but it still has many fans, including the Princess Royal. Mixed school, 13-18, paying as much as pounds 14,241.
Founded in 1934 to promote the virtues of the outdoor life. All pupils spend one week on school's sail training vessel, Ocean Spirit, whatever the weather. Students required to join groups including the coastguard, fire service and mountain rescue. Plays down the macho image these days, but still not for wimps.
ST CHRISTOPHER SCHOOL, HERTS
Co-educational boarding and day school. 530 pupils from two to 18, boarders start at eight. Fees up to pounds 12,564 .
Trendy "green" school. Vegetarian only. No uniform.
RUDOLPH STEINER SCHOOLS
There are 780 Steiner schools around the world, 26 in Britain and Ireland, plus 1,500 kindergartens. Fees between pounds 2,058 and pounds 4,563.
The Fellowship has provided an alternative system of education in the UK for more than 75 years. Believes that happy children perform the best. Stress on "tolerance and respect for oneself".