I got my first taste of our two-tier education system before my son Samuel, who is now 15, had even started school. Samuel has spina bifida, and I was told before he was born that the challenges we would face would be great. But I hadn’t anticipated it would mean a two-year fight to get him a place at our local primary school, where his big sister was already in class. I had to stand down as a governor to fight for Samuel’s inclusion, but we won, and by Samuel’s first sports day the school had started to change.

That day the PE teacher lined Samuel up, in his wheelchair, for the 100m race. The starting gun was fired. And within about 15 seconds it became apparent that the teacher had not thought through how Samuel, then the only child in the class with a wheelchair, would “run” this race. As I anticipated when I saw him on the start line, every other child crossed the finish before Samuel’s little chair had taken him halfway down the track. As Samuel pushed the joystick on the chair as far forward as he could, cheers from the parents on the sidelines lapsed into silence. Until someone started to chant his name: “Samuel! Samuel!” Within what felt like moments, everyone was cheering him across the finish line.

That moment encapsulated for me just how important it was for Samuel to be in that school. In that moment every parent, teacher and child knew it was the taking part that mattered. Ultimately, that is what inclusive education is about. While the needs of every child may be different, their place in a classroom – or on a race track – is valuable.

Jonathan Bartley challenging David Cameron about inclusive education.

But that place is increasingly under threat. And it has been for seven years. In 2010 I challenged David Cameron, on camera, on how his plans for disabled children would harm inclusive education, and my fears have been proven right. The number of children with special needs being home schooled has jumped by 57% in five years, while 1,000 children with recognised needs are waiting for a school place.

Adam Boddison, chief executive of the charity the National Association for Special Educational Needs, said schools are finding it difficult to be inclusive in part because of pressures on their budgets.

In August the UN released a damning report that warned Britain is failing to uphold disabled people’s rights. It painted a grim picture of the reality facing disabled people every day, with poverty rife and the right to independent living ignored.

When it comes to education, the UN confirmed what parents of disabled children already know: that Britain has a two-tier system. Not only has progress towards inclusive education “stalled”, the UN said, but disabled children are increasingly segregated, with the number of children with special educational needs (SEN) attending state-funded “special schools” rising. The government is in fact acting in direct opposition to its commitment to inclusive education. In March this year it announced that £215m extra funding for local councils for children with SEN can now be used for “special units” or “special schools”. The government didn’t even issue the UN’s guidance on inclusive education to local authorities.

You don’t have to look very far back in history to see how hard fought the battle for inclusive education was. Just 100 years ago disabled children were taken away from their families to be shut away in what came to be known as “exile schools”. They were treated like prisoners and taught only certain trades, such as basket weaving for blind children, which neither matched their potential nor were sufficient to pull them out of poverty. As recently as 50 years ago educating disabled children in mainstream schools was still almost unheard of, and abuse still commonplace.

You don’t have to look very far back in history to see how hard fought the battle for inclusive education was

Overcoming the stigmatisation of disabled people has been, and still is, a long battle. From 1972, when Paul Hunt had a letter published in the Guardian calling for equality for disabled people, to the formation of the Alliance for Inclusive Education in 1990, years of work preceded the 2009 UN treaty that finally secured inclusive education for disabled people as a right.

Simply placing Samuel in a mainstream school did not mean that his needs were always met. Inclusion does not automatically bring integration. But the year after Samuel “ran” that 100-metre race, the school introduced boccia, a precision sport that is played seated, to sports day. This was just one of many examples of how Samuel’s presence in the school gave other children the opportunity to develop skills that they wouldn’t have otherwise.

Part of a rich education is being exposed to, and learning how to deal with, difference. And realising that when we do, we are all the better for it.

• Jonathan Bartley is co-leader of the Green party