In the medical comedy TLC, doctors discovered a genius way to meet targets for cutting NHS waiting lists. They printed their patient list, chopped it in two with scissors, and stuck half of it in the bin. List cut. Job done.

Scriptwriters who want to parody the school system may be tempted to copy the joke. A stressed headteacher, seeking to improve the school’s pass rate, rounds up the worst pupils, slings them in a wheelie bin and trundles them towards a dustbin van. This might be funny, if it wasn’t so perilously close to reality.

St Olave's teacher: 'Weak students are treated as collateral damage' Read more

The Guardian’s revelation that St Olave’s, a grammar school in London, asked around 16 pupils to leave halfway through their A-levels because they were not getting B grades in mock exams has shocked readers. It was so widely shared it topped the most-read list on a day when North Korea was launching missiles and the largest state in America was partly underwater.

It’s not the first time this has happened. For example, three years ago, students at the London Academy of Excellence, a selective free school for 16- to 19-year-olds, reported they were asked to leave after failing to achieve sufficient AS-level grades that would get them into a top university. Yet, on results day, regular as clockwork, a spokesperson for the school called us at Schools Week, pushing the school’s impressive pass rates and high proportion of pupils attending Oxford and Cambridge. Fewer dud grades means a higher pass rate, which means better marketing, and more praise for the school’s leaders.

That said, post-16 education occupies a strange grey area. Access to A-levels is not automatic. Most sixth forms have minimum requirements before pupils can sign up, and St Olave’s and the London Academy of Excellence might argue that not signing pupils into a second year of A-level study is equivalent to not letting them into the course in the first place. Lawyers acting for the families say this practice amounts to an illegal exclusion by the school’s sixth form.

We also see this sort of exclusory behaviour happening lower down in schools – albeit less blatantly and to less uproar. Warwick Mansell has been investigating the problem of “missing children” for the past three years. He noticed that schools lauded for a successful turnaround often shrank their intake by around a fifth in a short period of time. While around 7,500 pupils were leaving secondary schools each year, only around 2,000 were reappearing. Where did the missing children go?

One hypothesis is that heads encourage parents to remove a low-attaining child and home educate them. By doing so, the school gets the low grades off its data and the parents avoid a formal exclusion for their child that goes on the record. This may explain why home schooling has doubled in the past seven years – with pupils heading towards exams the most likely to be removed.

Unfortunately outcomes for pushed-out pupils are pretty poor. The Who’s left report, published earlier this year by research gurus Education Datalab, estimated that around 20,000 pupils leave mainstream schools to go into alternative education – most of whom suffer poor outcomes. Only around 6% of these children achieve even five C grades at GCSE.

But excluding low-attaining pupils is good business for a headteacher trying to hit high pass rates. As Datalab researchers noted, exclusions have a “very flattering impact on the league table results of a school”. Hence Rebecca Allen, who heads the research group, has urged the government to ban schools from wiping a child’s data from their books. Instead, she wants the results of pupils who leave to count towards the schools’ final pass rate. “If schools were forced to keep the results of pupils who leave, then GCSE pass rates would fall by up to 17 percentage points in some cases,” the report notes.

Should one keep a child who is falling behind, or is it better they are educated elsewhere?

An alternative approach is banning exclusions altogether. Drew Povey, a headteacher who features in the new Educating Greater Manchester series, due to start on Channel 4 this week, has a firm no-exclusions policy. “Giving up on kids is the worst thing you can do,” he says, “I can’t stand it.” Instead, the school has an extensive behaviour management system and staff trained in therapeutic interventions. It had gone from excluding a pupil every fortnight in 2003 to not having excluded a single child in three years by the time of its last inspection report in 2013.

Keeping difficult children on the roll undeniably affects a school’s pass rate, however. And this is where the moral issue gets difficult. Who is a headteacher responsible to? Should one keep a child who is falling behind, or is it better they are educated elsewhere? Is a reputation for great results more important to the wider community than honouring a particular child’s right to continue studying?

In the end, the courts will decide. Lawyers acting on behalf of the St Olave’s pupils will put their case in front of a judge later this month. And school leaders who have been pondering throwing kids on to the scrapheap in a bid to improve results would do well to rethink. There’s no comedy in ruining children’s lives.

• Laura McInerney is editor of Schools Week. She previously taught in London for six years