Mock exams are over and many pupils will be breathing a sigh of relief that the parents’ meeting that follows is behind them. Coursework and controlled assignments may be their priority now but parents should be aware that they might need to be addressing different priorities.

Firstly, let us explode the urban myth that pupils do one grade better in their real GCSEs than in their mocks. This reassuring myth is just that, a myth. The truth is that pupils are most likely to gain the same grades in August unless something changes significantly before June.

In some subjects this may be the introduction of coursework marks, in other cases pupils undergo a Pauline conversion and work much harder than previously. The likelihood is that most pupils will perform similarly in the two sets of exams.

So if your son or daughter is below the borderline for Sixth form admission following mock exam results, there is a real possibility that they will be culled by their school in late August when headmasters and headmistresses review the GCSE grades gained by their pupils and decide whether to admit them into their Sixth Form or cast them aside.

Lucky parents may get a sympathetic phone call from an apologetic teacher who will make them aware of the impending crisis; others will merely receive a standardized letter informing them that their child has failed to meet the threshold for entry into the Sixth Form. “Push off elsewhere, you’re a threat to our League Table position” is the message.

Every Public School website will trumpet the strength of its community, the breadth of activities on offer, how highly they value the development of all-round talent and “the whole person.” All this is true, until the Annual Cull. Then, on a single day those lofty aspirations are swept aside amidst excuses that “our Sixth Form curriculum is too demanding”.

It no longer matters that James manages the lighting in the school theatre, that Rosie has cycled 500 miles for charity or that William leads a Reading Club in the local nursery. They haven’t gained five Bs or eight As and they aren’t wanted any more.  Game Over – Push Off.

The most ruthless schools repeat the exercise at the end of Year 12, midway through their A level course, placing pupils under constant pressure to perform or face expulsion. This is bad educational practice which takes no account of a pupil’s mental or social welfare.

Such practice is bad enough in day schools where, despite a change of school, friendships and extra-curricular activities can still be maintained within the locality. But this pales into insignificance compared to the dislocation when pupils are dismissed from boarding schools.

Years of friendship which have been built through shared endeavour and experience are severed with a single letter. Imagine the horror for parents who spent months agonizing over the choice of school, now blaming themselves and worrying about where to go next.

The worst of this practice is that schools know exactly what pupils are capable of well before admitting them. From the age of 11, children face a barrage of aptitude, verbal and non-verbal reasoning tests before entry to senior school, all of which provide an accurate prediction of GCSE outcomes. There can be no excuse that “George is finding the work more difficult than we forecast.”

The Cull is a cynical device employed by schools with league table ambitions that go beyond the ability profile of their recruitment pool.

Andrew Fleck is Headmaster of Sedbergh School

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