Schools should be fined if they are "gaming the system" by informally excluding low-achieving pupils ahead of the exam season in order to preserve their league table scores, the children's commissioner for England has said.
Anne Longfield, who is charged with promoting and protecting the rights of all children in England, said it was "increasingly clear" some schools were taking certain children off their rolls because "they think they won't get good results".
Speaking to The Observer, the commissioner said: "Any school that does this is abandoning their responsibility to children, passing the buck to others who are often ill-equipped and don't have the support they need to provide a good education.
"As a result, very vulnerable children are falling through the gaps in the system, increasing the chances they will then go on to lead difficult adult lives.
"I will be calling on the Government to set out what measures it will take, including looking at the possibility of financial penalties for schools, to ensure this practice stops now."
Education campaigners have long expressed concern at the notion of off-rolling - where problematic, naughty or academically poor pupils are unofficially removed from the school.
This can include a "managed move", when a school looks for an alternative to expelling a pupil, such as asking another institution to admit the youngster to give them a new start.
Other examples, identified by local authorities, include suggesting the pupil is educated at home for an unspecified period, prompting concerns children could fall through the gaps and out of the education system.
Under current laws, a school can only exclude pupils, either permanently or for a short period of time, for disciplinary reasons - not because of their attainment or ability.
A Department for Education spokesman said: "Any decision to exclude should be lawful, reasonable and fair.
"While exclusion can be used as a sanction for schools to deal with poor behaviour, permanent exclusion should only be used as a last resort, in response to a serious breach, or persistent breaches, of the school's behaviour policy and where allowing the pupil to remain in school would seriously harm the education or welfare of the pupil or others.
"We have announced that we will be launching an externally led review of exclusions practice aimed at improving practice in exclusions through sharing best practice nationwide, focusing on the experiences of those groups who are disproportionately likely to be excluded."