Legal action is being pursued after a grammar school withdrew places for a number of A-level students who failed to achieve certain grades.
Simpson Millar solicitors have initiated judicial review proceedings against the governing body of St Olave's grammar school in Orpington in south east London after two students were told they could not continue their A-level studies into Year 13.
The pupils, who cannot be named for legal reasons, failed to achieve Bs in any of their subjects taken in the first year of sixth form, lawyers said.
Days ago they were told they could return to the school but only to study a BTEC in health and social care.
The high-achieving school, which will start its new term on September 5, is said to operate a three-pronged policy to "maintain its exceptional A-Level results".
Year 12 pupils will normally have gained three Bs or higher if they wish to complete their studies in Year 13, the school's sixth form rules and regulations state.
An email sent to the school's Year 12 tutors in June said that pupils who scored a C would not be able to pursue that subject through to A-level.
If a student scored a C, they must sign an agreement that the school reserves the right not to enter them for A-level examinations in any subject in which it is considered they will not score a B or above.
The claimants are contesting the policy on the grounds that it does not "safeguard and promote the welfare of the children at the school, and is irrational".
It is unlawful, they say, because "the school does not have the power to exclude pupils between Year 12 and Year 13 due to an alleged lack of academic progression".
Dan Rosenberg, the lawyer acting for the families, said: "Proceedings have now been issued. As well as assisting our clients, these proceedings will hopefully assist others who may be affected now and/or in the future by the school's policies and practices.
"We hope that 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."
Education lawyer Imogen Jolley, of Simpson Millar, said Department for Education exclusion guidance covered local authority maintained schools, such as St Olaves.
She said: "In maintained school sixth forms there may be grade boundaries to get in, but once you are in the only way they can get rid of you is if there is a behaviour issue. It would be unlawful to exclude a pupil for a reason such as academic attainment or ability."
The Department for Education said it would not comment while legal proceedings were ongoing, but issued guidance on its exclusion policies.
Neither the school, nor the London Borough of Bromley, which is listed as an interested party, responded to requests for comment.
A hearing to determine whether a judicial review will be granted is due to take place on September 20.
If granted, the hearing will proceed that day.