Schools avoid Ofsted inspections of their worst classes, raising doubts over assessments of behaviour standards, MPs have heard.
Schools put cover teachers into classes to avoid Ofsted inspectors, the education select committee was told.
The watchdog's verdicts on behaviour "aren't worth the paper they're written on", said Tom Trust, formerly of the General Teaching Council for England.
Ofsted said there was little evidence that schools try to mislead it.
Mr Trust, formerly an elected member of the GTC's council, told MPs he did not believe headteachers' evidence on behaviour standards in their schools was reliable.
He said he was aware of a head teacher who claimed there were no disciplinary problems in their school despite lesson observation notes recording numerous "misdemeanours" in the classroom.
Mr Trust claimed that inspectors would be less likely to assess lessons taught by cover teachers.
Others giving evidence to MPs shared doubts over the reliability of assessments of behaviour, including Katherine Birbalsingh, the teacher who gave a controversial speech at this year's Conservative party conference.
She said she had only ever worked in schools classes rated as good and outstanding and "Ofsted's standards are not high enough when it comes to behaviour".
And Sue Cowley, educational author, suggested that some schools might send disruptive pupils home for the period when Ofsted inspectors are due.
"If you want schools to give you an honest picture of what's going on, day to day, you can't expect all lessons to be outstanding. Some days, teachers are knackered," she added.
However, Paul Dix, behaviour management trainer, broadly accepted Ofsted's judgements on behaviour.
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 - meaning that nearly 80% of schools were considered good or outstanding.
Mr Dix said effective training was key to improving behaviour in schools, while Ms Birbalsingh said senior staff in schools needed to be "held to account".
She also said that both teachers and school management staff should be sacked if they failed to control their pupils.
"If people don't do their jobs, they need to be fired," she said.
Schools Minister Nick Gibb said behaviour was one of the four key areas Ofsted will be asked to look at in future, rather than the current 17.
The government has outlined plans to allow teachers to give no-notice detentions and to broaden their powers to search pupils for forbidden items
"We have set out very clearly that we want to give teachers and head teachers the powers and support that they need," he said.
The government also plans to clarify and strengthen guidance on teachers' rights to physically restrain disruptive pupils, he said.
An Ofsted spokesman said that schools can currently be inspected with only two days notice and "there is very little evidence that schools try to mislead Ofsted".
"Inspectors also consider the views of parents, talk to pupils about behaviour in the school and examine documentary evidence about, for example, the number of exclusions. Inspectors will quickly identify schools where behaviour is poor," the spokesman said.