Laywers claim the pair at St Olave's in Orpington were told they could not embark on their second year of sixth form after they did not secure at least three B grades.
Simpson Millar solicitors have initiated judicial review proceedings in a bid to change the rules and regulations at the grammar school.
A lawyer acting for the families, Dan Rosenberg said: "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."
St Olave's operates a policy to maintain what it calls its "exceptional A-Level results". Dating back to 1571, it has sent a significant number of pupils to the country's top universities.
Earlier this month, the school announced that 96 per cent of its A-Level students achieved grades which ranged from A* to B.
Under its rules, students are usually expected to achieve at least three B grades during the lower sixth year in order to progress to upper sixth.
Legal representatives for the families of the two students argue, however, that it is unlawful for a school to exclude students on the grounds of an alleged poor academic achievement.
Education lawyer Imogen Jolley from Simpson Millar 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, London Borough of Bromley and the school itself all declined requests for a comment.
A hearing on 20th September will determine whether a judicial review can take place.