Headteachers are calling for the government to reform school league tables to prevent the "perverse incentives" that lead to a minority of schools "off-rolling" pupils.
Geoff Barton, general secretary of the Association of School and College Leaders, said such practices were never acceptable, but that it is "is simply a fact that a small number of rogue results can send your Progress 8 score into nosedive", creating incentives for schools to act unethically.
Need to know: What is off-rolling?
He is due to make his call for league table changes at the second annual ethical leadership summit in London today.
Mr Barton will argue that while "there are two sides to every story", "gaming" and "off-rolling" – where pupils are removed from rolls, without exclusion, to boost a school's results or reputation – "are just plain wrong".
The heads' leader says that, while such practices have "no place in education", "we also need to consider the fact that these are not practices which happen randomly out of a clear blue sky".
"They are closely associated with the topsy-turvy way in which the performance of schools is measured and judged," he will say.
“In our system, it is simply a fact that a small number of rogue results can send your Progress 8 score into nosedive.
"It is a perverse incentive to do the wrong thing and ease out the pupils in the margins. These will inevitably be the young people who need the greatest support – vulnerable children who are struggling to cope.
"And when the stakes are so high – when careers and reputations hang in the balance – the temptation to find a way of gaming performance tables is also that much greater...
"Wouldn’t it be better and more sensible to have performance measures which incentivise and reward good behaviour rather than expecting schools to do the right thing in spite of the system? And would that really be so hard to do?"
Mr Barton suggests school performance tables that assess schools on a wider range of measures, such as how well schools support their most vulnerable pupils, how well they collaborate with other schools for the benefit of all children in their area, or the extra-curricular activities they offer their pupils.
He said this would be "more useful and informative for parents" than the current system's narrow focus on exam results.
“Reforming the performance system in order to get rid of perverse incentives to do the wrong thing isn’t something in our power – it is a job for government and it must act," he will say.
In October, it was reported that one in ten pupils had had an unexplained move out of their secondary school. For the cohort that sat GCSEs in 2017, there were 69,000 unexplained exits during their five years of secondary education.
And in July, Tes revealed how the parents of one autistic pupil were told by the child's headteacher that the school system was a "one-size-fits-all sausage factory" and were encouraged to educate their daughter at home.
The Department for Education has been contacted for comment.