In August, three Wandsworth schools were told by the adjudicator Professor David Newton to cut the number of pupils they selected.
Two of the schools, which have foundation status and are not under local control, decided to cut a total of 31 selective places from September 2004.
But Wandsworth council took the case of the third school, Ernest Bevin, an LEA-controlled technology college, to a judicial review.
On Monday at the High Court, Mr Justice Goldring upheld the council's case and quashed the adjudicator's decision to cut the proportion of "academic ability" places at Ernest Bevin from 33 to 30 per cent of the intake.
Although this adds up to just six children, the school and LEA were determined to preserve the status quo, after a series of adjudications restricting admissions arrangements in the borough.
The Office for the Schools Adjudicator said it would not be appealing. The move to cut selective places followed complaints that selection left neighbouring schools unable to achieve a balanced intake.
But Mr Justice Goldring said partial selection at the three schools "was not a sufficient and primary cause of unfairness".
Wandsworth has called for the adjudicator's role in school matters to be abolished.