Some secondary schools are failing teenagers from deprived backgrounds by giving them reading material for primary-age children, the head of Ofsted has warned.

Amanda Spielman said she had been angered to find schools setting lower expectations for children simply because of their background. It follows evidence that students as old as 14 are being given English texts designed for primary children.

In a wide-ranging interview with the Observer, Spielman said she had been driven “absolutely nuts” by evidence that some schools were assuming that disadvantaged children were “automatically low-achieving and need a watered-down curriculum”.

“We have [seen evidence this is a problem] in various ways,” she said. “One thing that has been pointed out in various contexts … recently is the tendency to use texts, even in secondary education, that only require a reading age of 10 or 11.

The Observer view on the school system that does little to help our poorest pupils | Observer editorial Read more

“Children can go all the way through secondary school and then go bump when they hit real demands in post-16 education or have aspirations for university, because they just don’t have the experience or practice of reading more demanding texts. Schools can think they are being helpful by adapting and providing relevant material. But in fact it hollows out education and means that disadvantaged children don’t get the experience that they absolutely should. The job of schools is to make sure that children get the things they won’t necessarily get at home.”

Spielman, who took over as chief inspector last year, warned that schooling standards would be put at risk by further cuts to Ofsted’s budget. She said the inspectorate was now at the “very limit of giving the level of assurance that we are expected to give”. It comes after Ofsted was criticised for failing to inspect some schools for up to a decade and only spending a day inspecting others.

The chief inspector also warned that the schools system had “stopped acting in children’s interests” by allowing thousands of underperforming pupils to be ushered out of the door to protect their school’s exam pass rates.

She said some schools were putting themselves first and students second by shuffling out underperforming pupils in a process known as “offrolling”. Ofsted found that between 2016 and 2017 more than 19,000 pupils did not progress from year 10 to year 11 in the same state-funded secondary school. Many were disappearing from schooling altogether.

Facebook Twitter Pinterest Between 2016 and 2017 more than 19,000 pupils did not progress from year 10 to year 11 at the same state-funded school. Photograph: Graeme Robertson/The Guardian

“It’s an example of where, to some extent, it looks as if the schools system has stopped acting in children’s interests, that schools’ interests are getting put ahead of children,” she said. “We need inspectors to know where a school is losing children at a rate that looks suspiciously high and have a tough conversation with the school where it is happening. There are about 300 where we can see that the number of children leaving between years 10 and 11 looks too high to be statistical noise.”

Spielman said the practice was creating a system in which challenging pupils ended up in underperforming schools. “When you have children falling out of one school, what is the school that they will get put in if parents or local authorities are looking for another? It will be the undersubscribed school, which will typically be the school that is doing badly, often in a tough area.”

She suggested the practice was adding to her concerns about children supposedly being educated at home.

“We found that half of the children who come off a mainstream school roll at the end of year 10 don’t appear on the roll of any registered provision for year 11. That is a very big number disappearing. Home education generally is something I’m really concerned about. Those children disappear into the ether.

“Nobody really knows what they are doing, who is taking responsibility for them. At one level parents are supposedly taking responsibility, but is that what is happening? There is a militantly independent group of homeschoolers who have existed for a very long time, who are politically quite powerful. There is a growing religious strand of people who want to protect their children’s faith and make sure that everything about the child’s education experience happens within the faith.

“Sometimes that is in registered schools, but probably though it is hard to be sure - some children who are supposedly being home-educated are in fact in unregistered religious schools.”

Spielman, who has faced hostility for accusing faith schools run by religious conservatives of deliberately resisting British values, said she believed some groups were deliberately finding loopholes in the rules to ensure their child’s education was not inspected.

“Home education takes us into the territory of stuff that is hard to discuss that a lot of people find ‘undiscussable’,” she said. “We are trying to explore what is happening and why it brings you up against people who really do not want their children’s education scrutinised. I’m trying to make sure that we talk about the tough stuff and that we help make the tough stuff discussable, so that people don’t run away from it. It is difficult and it has to be talked about.”

Spielman said there must be no return to the time when concerns about “cultural sensitivity” led some local authorities to be reluctant to intervene in cases of child sexual exploitation. Ofsted also inspects children’s services offered by local authorities.

“One of the biggest failures of children’s services in recent years hasn’t been about resources, it has been about issues such as child sexual exploitation being undiscussable, because of different kinds of cultural sensitivity,” she said. “At Ofsted, it is easy to see how people slip back into being anxious not to offend communities or tread on toes. We must make sure we keep up the pressure to talk honestly about the tough stuff.”

She responded to criticism of the regularity and thoroughness of Ofsted’s school inspections by warning that the inspectorate was doing everything it could to keep up school standards. “We are at the very limit of giving the level of assurance that we are expected to give on the current budget,” she said. Asked if further cuts would damage standards, she said: “I would hope that is how it is seen.”