Headteachers are hiding unruly pupils from Ofsted inspectors to stop their school being labelled as badly behaved, a cross-party group of MPs heard today.
Naughty pupils are suspended, or supply teachers brought in to cover disruptive classes to stop inspectors seeing the worst children, the Commons education select committee was told.
Headteachers cannot be relied on to provide evidence of bad behaviour, because they have too much to lose, it heard.
Tom Trust, a former elect member of the General Teaching Council for England (GTCE) for the secondary sector, said he was aware of a headteacher who claimed there were no disciplinary problems in their school despite lesson observation notes showing numerous "misdemeanours" in the classroom.
He told MPs: "Getting evidence from headteachers is not always reliable because they have got a lot to lose."
Speaking about his own experiences, Mr Trust told the committee: "I did some supply in a school once that was having an Ofsted report and I went in and I got my supply list for the day, the lessons I was going to cover, and I was told 'Actually, these teachers aren't away but you're going to go in there with them anyway'.
"In I went, and I later found out it was very unlikely that Ofsted inspectors will go into a class that's being covered by a supply teacher, it's not impossible, but very unlikely.
"On each morning, the Ofsted inspectors were given the little file of cover supply. They knew which lessons were being covered and they thought the ones I was supposed to be covering were being covered.
"They weren't and they were terrible classes, not necessarily with weak teachers, maybe some of them were, but they were terrible classes full of really disruptive pupils."
He added: "Ofsted's views on behaviour are not worth the paper they are written on in my humble opinion, because there are lots of strategies that headteachers use to avoid the Ofsted inspectors seeing the worst children.
"That may shock you and you may think that's just an isolated incident but it's not, it goes on."
Sue Cowley, an educational author and trainer said school Ofsted inspections should be done on the same basis as those for nurseries, with inspectors turning up unannounced.
"If you want an accurate picture, if you don't want schools to exclude pupils for the week, you want to get it down to somebody turns up," she said.
More than one in five schools (21.3%) was judged to be either "satisfactory" or "inadequate" in terms of pupil behaviour by Ofsted inspectors last year, according to statistics published by the then Department for Children, Schools and Families (DCSF) in April.
And a survey published by the Association of Teachers and Lecturers (ATL) earlier this year found that teachers believe behaviour in schools is worse than it was five years ago, with pupils as young as five being disrespectful, intimidating and violent.
Almost four in 10 (38.6%) teachers said they had encountered physical aggression in the classroom, with more than a quarter (25.9%) saying the violence was directed at them.