An invitation to take part in the BBC series Prescott: The Class System and Me seemed too good an opportunity to miss. I thought it was about time that Mr Prescott, the old class warrior who regularly attacks independent schools, should come and see one for himself. So I invited him to Rugby.

The head of an eminent boarding school and a man who thumped a protester - what on earth could we have in common? Quite a lot, actually. Both of us started life with limited outlooks that brightened considerably thanks to education. For me, it was the chance to get off a training ship and go to boarding school with the help of a bursary. For him, it was a spell at Ruskin College, Oxford. In both cases, the experience transformed our lives.

My father was a soldier in the ranks and after he left the Army my parents moved to a council estate in the Scottish Borders. When I was 12 I was sent to school on a training ship, the Arethusa, run by a charity. The idea was that I would join the Navy at 16. But when the ship closed I was given the opportunity to go to Pangbourne College on a bursary. Schools of that kind were a very remote concept indeed, but the experience transformed my life. The school gave me the support and opportunities I needed. Instead of leaving at 16, I studied A levels and read history at Cambridge.

I can say with all confidence that without that opportunity I would not now be sitting at the desk of my celebrated predecessor, Thomas Arnold. Similarly, Mr Prescott's time at Ruskin College propelled him on his own extraordinary journey from ship's steward to the Queen's Second Minister. We are living proof of that great Chartist slogan that education is a liberating force.

I thought Mr Prescott's visit would be enlightening for both parties. I was wrong. While he was extremely good company, the encounter was ultimately disappointing. He wrapped himself in the old Labour mantra like a much loved but rather dated coat and refused to listen. Did the presence of the TV crew prevent us from budging his prejudices? He was certainly more relaxed without them, but in the end they made no difference. Even after six hours of talking to pupils and staff, it was clear that he was not for changing.

Nonetheless, now that the cameras have stopped rolling, I would like to remind him that his hopelessly outdated view of independent schools has no bearing on reality. The independent sector is a much broader church than he thinks. It is a myth that only rich toffee-nosed children attend schools such as Rugby. Nearly a quarter of pupils at Independent Sector Council schools come from postcodes where people live on average or below-average incomes. Around one in three receives some financial assistance. Far from being a barrier to social mobility, many independent schools encourage it. Efforts to widen access are not, as Mr Prescott assumes, a knee-jerk reaction to the new public benefit test. In common with many other independent schools, widening access is one of Rugby's founding principles. This commitment was made when it was established in 1567 and remains at the very heart of what the school is doing now.

We were the first school to cap scholarships at 10 per cent for those who can afford the fees, while increasing them to 100 per cent for those who cannot. For hundreds of years, Rugby has provided financial assistance for local children and I wanted the same opportunities for boarders. Over the past five years the Arnold Foundation for Rugby School has enabled 35 boys and girls from deprived areas to board at the school. Working with educational charities, we have identified children with the most to gain from the opportunities of a boarding school. Within ten years, we aim to fund full fees for 80 pupils - 10 per cent of the school - through our bursary schemes.

But a good education is not about money, it is about values; about fostering self-motivation, discipline and aspiration: qualities that can be found in any home and any school, regardless of sector or class. If there is a division, it is not between classes, but between good schools and bad.

Everyone would like a stronger state sector, but Mr Prescott and others are wrong to blame its flaws on independent schools. In attacking private schools, he attacks every parent's right to choose the best for their children. Many parents choose a school like Rugby for the values it instils. We are free to experiment and to think outside the box. Most importantly, we are free to adapt our curriculum to the needs of the pupil. It is these principles and freedoms, that are not the preserve of the wealthy or the upper classes, that make our independent schools the envy of the world.

If only Mr Prescott recognised that what he saw at Rugby - and which is replicated throughout the independent sector - is something to celebrate, not punish. Criticising what works is no way to fix what doesn't. The greatest schools enable pupils to be the best they can. There is no shortage of them in both the independent and state sector. What is important is to learn from what they have to offer. By imposing his class system on education, the former Deputy Prime Minster entrenches divisions between state and independent schools. If only he could see what all of us in the independent sector see - that our schools are powerful engines of social mobility.

Patrick Derham is Head Master of Rugby School. Prescott - The Class System and Me is on BBC2 tonight at 9pm

Times Recommends

• Fragility always gets my sympathy vote
• Double it and you get the real jobless total
• Has al-Megrahi paid his council tax?