A high-achieving grammar school is facing a legal challenge over whether pupils can be stopped from progressing into the final A-level year.
Parents of pupils from St Olave's, in the London borough of Bromley, say that removing pupils between Year 12 and 13 - the lower and upper sixth - is a form of exclusion.
Lawyers representing the parents claim that such an exclusion is unlawful and are seeking a judicial review.
The school has not commented so far.
Top grammar
St Olave's is one of England's top-performing grammar schools, with places decided on academic ability.
But parents are contesting whether pupils who have been admitted to the lower sixth should be able to be stopped from continuing into the upper sixth and taking their A-levels.
Their lawyers are seeking a judicial review, with the application to be heard on 20 September, in a dispute first highlighted by the Guardian.
The parents claim that preventing pupils from continuing into the upper sixth year is in effect an exclusion - and it is unlawful for a school to exclude a pupil on the grounds of a lack of academic progress.
Dan Rosenberg, the lawyer representing the parents, said: "We hope the governing body will now reconsider the policy and its application, and further that the children affected can return to their A-level studies at the start of term with their peers."
Any change as a result of the challenge could have far-reaching consequences, because many schools put conditions on pupils carrying on into the final year.
From the school's perspective, this would not be seen as an exclusion, but a decision on whether to admit a pupil for entry into the upper sixth and A-levels.
Professional judgement
Head teachers say that this is about professional judgement - and that they need to be able to decide whether pupils are suitable for a qualification.
But there have also been claims that removing weaker students at the end of lower sixth is a way for schools to protect their standing in league tables.
The legal challenge facing St Olave's would limit the school's right to carry out such selection between the two years of sixth form.
It argues that academic selection can be used when pupils enter the school, but not for progression between years.
The legal challenge claims that pupils were expected to achieve a grade B or above in three out of four AS-levels.
This year's A-level results at St Olave's saw 75% of all grades being awarded at A* or A and 96% were at A* to B grades, far above the national average.
