Some schools in England are illegally excluding pupils, sometimes permanently, without going through the full formal process, a report says.
England's children's commissioner Dr Maggie Atkinson heard some pupils were moved to other schools or sent home without an exclusion being recorded.
Most schools tried to hold on to troubled pupils, she said, but a minority excluded on "a whim".
The Department for Education said unofficial exclusions were unlawful.
The inquiry found much evidence of schools and colleges working together and with other agencies to ensure the best outcomes for children.
But it also heard evidence that in a small minority of schools, heads were excluding informally, illegally and too readily.
'Under the radar'
One extreme example involved a head teacher admitting to inquiry researchers that he was planning to send a group of final-year GCSE pupils home from Christmas until May.
He told the researchers: "We will get their parents in and ask them to keep their children at home for the rest of the academic year; otherwise it's permanent exclusion.
"The pupils are coded as 'C' (authorised absence) and slip under the radar."
The report,They Never Give Up on You, says: "This practice is illegal and simply unacceptable. However, because it is usually covert and informal with no records kept, it is extremely hard to identify and quantify."
It adds that DfE guidance is clear and unequivocal that "formal exclusion" is the only legal method of removing a child from school, whether it be for part of a day or longer.
However, many parents do not complain about such treatment because they are often unaware of the rights of their children, it says.
Dr Atkinson said: "We knew a minority of schools excluded informally and therefore illegally, but for the first time in this inquiry we have this on record.
"Whilst most schools work far beyond the call of duty to hold on to troubled and vulnerable children, a minority exclude on what seems to the observer to be a whim.
"And for whatever reasons... we have not sufficiently challenged the failures and brought about the changes required. We must do so now."
The report accepts the need for pupils to be excluded from time to time, but it warns that this should only happen when they are harming themselves or others, or if they are preventing others from learning.
It calls for the government to investigate the full extent of unlawful exclusions in England's schools, and to recommend measures to prevent them from acting in such a way.
And it recommends that - if Ofsted finds evidence of unlawful activity during an inspection - the school should be graded "inadequate".
The children's commissioner also warned that changes to the way children and their parents can appeal against exclusions were potentially in breach of the European Convention on Human Rights.
This is because the exclusion review panels introduced by the 2011 Education Act do not have the right to insist on reinstatement where they rule in favour of a the child, as they did previously.
The report calls for the Act to be amended so that successful appeals lead to automatic reinstatement.
The National Association of Head Teachers general secretary, Russell Hobby, said inclusion was at the heart of most head teachers' professional pride, and that even "essential exclusions" were regarded as failures.
"Unfortunately, under the relentless pressure of a target-driven culture which cares only about results, not how you achieve them, a very small number fall short of this ideal and use covert measures to alter their intake," he added.
A Department for Education spokeswoman said: "Unless there is good behaviour in schools teachers cannot teach and students cannot learn.
"Schools need to be able to exclude disruptive pupils as a last resort - to enforce non-negotiable school rules, to protect staff and students and to guarantee that excluded children get the support they need."
She continued: "Our policy on exclusions is the right one, and we will continue to support it. Obviously unofficial exclusions are unlawful. All schools must follow the legal exclusion process."
Shadow Education Secretary Stephen Twigg said there was something systematically wrong if schools are breaking the exclusions law in this way.
"This is often at the expense of poor, black or special needs children.
"The Children's Commissioner has done some of our most vulnerable children a service in exposing the 'ghosting' of pupils from school to school and unrecorded exclusions," he added.
NUT general secretary Christine Blower said: "It makes far more sense to invest in keeping young people in school rather than having to get them back on track once they have been excluded, or in the worst cases lost to the system.
"However, for that to happen, schools need support and extra resources to manage effectively children's emotional and behavioural problems."