Schools are converting toilet blocks and classrooms to build isolation booths to accommodate “disruptive” children, the children’s commissioner has said, as campaigners warn that excessive use of the practice could be putting young people’s mental health at risk.

Anne Longfield said she had heard “horror stories” about children’s experiences in isolation booths – spaces in which pupils sit in silence for hours as punishment for breaking school rules and disruptive behaviour.

Some pupils told her they had been put in isolation repeatedly for days or weeks at a time and described the experience as “distressing and degrading”, while others said they had slept because there was nothing to do.

The children’s commissioner is conducting research to find out how widespread the use of isolation booths is and what kind of children are affected in response to mounting disquiet among parents and mental health campaigners about the practice.

At one school, Longfield said she had been told that a portable booth made out of cardboard was used to place over a child in the classroom. Pupils told her they had been put in isolation over minor uniform breaches or the wrong hairstyle.

Longfield is also concerned that isolation is being used by some schools as a “gateway” to exclusions and that it affects disproportionately high numbers of children with special educational needs.

Last week, the Centre for Mental Health charity warned that putting challenging pupils in isolation for extended periods at school could harm their mental health and that young people who had already suffered trauma were particularly vulnerable.

But some in the education sector back the the use of isolation for disruptive children. The government’s behaviour tsar for England, Tom Bennett, recently told the Guardian that “removal rooms” were used so children could learn in a safe, calm environment.

“They aren’t isolation rooms – I’ve never seen a school leave a child unattended. They’re supervised, and more often than not, accompanied by other students. So claims that these are isolation spaces is nonsense. These are safe spaces, run by adults,” he said.

Other headteachers say the use of well-run isolation rooms means lessons are disruption-free for other children. Ruth Robinson, a school leader from Swindon, explained on Twitter that at her school 10-15 out of 900 students on average spent the day in the school’s isolation room, though booths were not used.

“The impact of this approach is that every single lesson taught at the school is disruption-free. No teacher is ever talked over. Pupils are not rude. Pupils don’t refuse to work. This system is strict but it isn’t draconian. It works because it’s fair and because everyone understands it.”

In an interview with the Guardian, Longfield said: “I get lots of anecdotal evidence, lots of kids and some parents, telling me about the awful experience they have had in isolation and isolation booths.

“I get to hear about children who have spent long periods of time in isolation rooms. They have told us that they find it distressing and degrading. They said they feel like they are being warehoused in isolation, and they say sometimes it’s for days or weeks at a time.

“I was told of a school where they were converting a toilet section into isolation booths – and the comment there was it was very handy because they had already got the cubicles.”

Longfield said she had also heard of schools where teachers were having their classrooms taken away because more booths were wanted, “and I heard of one bizarre example of how they had a kind of portable booth which was a cardboard booth which they just put over the child wherever they were sitting”.

There are no official figures, but one BBC investigation said more than 200 pupils had spent at least five straight days in isolation areas in schools in England in 2017. A “Lose the Booths” conference has been organised for next weekend to encourage headteachers to remove booths and employ better practices.

John Procter of Corporate Office Furniture, which sells isolation booths, said about 50 schools had acquired units from him over the last five years. In the last three months alone, around eight schools have bought around 100 units, each costing £175.

“It’s not cruel,” said Procter. “These people who say it’s wrong, they need to spend a day in a school and see actually what these teachers have to go through. The teachers have not signed up to being abused.”

Many in the sector say a distinction should be drawn between isolation rooms and the use of confined booths, in which children have to face the wall and cannot communicate with each other. They say there is also a balance to be struck between the needs of challenging pupils and those who want to work without distraction.

But Longfield said she was concerned that there may be a link between the use of isolation and exclusions. “Often parents, especially those with children with special educational needs, will talk to me about how their children are always being put in isolation booths.

“And then after three or four or six months of the child having a terrible time at school, there’s a discussion with the parent about [how] maybe the school couldn’t offer the support that child needed. So I’m very worried about that gateway.

“Some children have said they even just sleep in the booths. In fact, one child even said: ‘Oh well, they’re not that bad,’ – they get to have a sleep.”

The government is due to publish fresh guidance later this year on managing behaviour and the use of isolation rooms and exclusions.

“All kids have a right to a good education and if they are being removed from a classroom and put into isolation they are not getting their education,” said Longfield. “I’m being told that some children are put in isolation because they have their hair in the wrong style or they have forgotten something or they are wearing some part of the uniform that isn’t right. I don’t want kids put into isolation about that.”

Longfield said she accepted there needed to be quiet areas outside the classrooms where children could calm down. “But what I’m concerned about is when those isolation booths seem to be booming in some schools and when those periods of isolation outside the classroom become very much the norm and when they are being abused.”

A Department for Education spokesperson said: “Schools may choose to use in-school units, whether that be to provide additional support to vulnerable pupils, or as a sanction to remove pupils with challenging behaviour from the classroom.

“Our guidance is clear, however, that use of isolation must comply with pupil safeguarding and welfare requirements. Pupils are not to be kept in isolation longer than necessary and their time spent there must be as constructive as possible.”