The judicial review into the Catholic school has been found in favour of Richmond Council.
Mr Justice Sales made the unusual step of making an announcement today because of the children awaiting confirmation of a place at St Richard Reynolds school.
Mr Sales stated the council had acted lawfully in its processes.
Leader of Richmond Council Lord True said: “I am delighted with today’s outcome which supports the clear, democratic decision that was taken locally in pursuit of the previously longstanding policy of both parties on the council. It will come as an enormous relief to the hundreds of families whose hopes for their children’s education has been threatened by this hostile legal manouvering.
“Over the past year, the British Humanist Association has elbowed its way into Richmond with its clear national agenda of hostility to faith schools – their action has been uncaring and unsympathetic to the many people within the Richmond Catholic community who have worked hard to bring their dream of a dedicated secondary school to fruition.
“It has also totally ignored the parallel action being taken by this Council to provide more places for all."
Richmond Inclusive Schools Campaign teamed up with the claimants British Humanist Association (BHA) to take the case to review.
Chief executive of BHA Andrew Copson said: "We are disappointed, but we will certainly consider appeal, if the full judgment, when we receive it, allows that.
"This [decision] obviously hinged on a technicality, but the wider principal that discriminating religious schools shouldn't have preference over inclusive schools is still an important and serious and we will never stop campaigning on that basis."
Justice Sales will make his full account of his decision in coming weeks but said Richmond Council had acted lawfully.
Paul Barber, from the Diocese of Westminster, said: "we are absolutely delighted. It's what we expected, but we are very glad to have this unnecessary uncertainty out of the way."
Interim governor for the school Andrew Cole said: "It would be been a great shame if the school had been delayed. This is fantastic news."