St Olave's in Orpington is facing legal action from parents after 16 pupils were told they wouldn't be able to finish their courses because of a failure to achieve three Bs in their summer exams.
Announcing the action on Wednesday, 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."
In a statement about the case, a spokesman for the Department for Education, said: "Schools are not allowed to remove pupils from sixth form because of academic attainment once they are enrolled."
A spokesman for the London Borough of Bromley said it would be inappropriate to comment while a judicial review is pending and directed queries to the school, stating that its governing body is the admissions authority which sets policy.
The school has not responded to requests for comment.
A hearing to determine whether a judicial review will be granted is due to take place on 20th September.
If granted, the hearing will proceed that day.