The Barnfield Federation has been told that it did not give people enough time to have their say on its new admissions arrangements.
The Schools Adjudicator also said that elements of the new arrangements did not meet government requirements, and has told the organisation to revise them “as quickly as possible”.
In July the federation announced it was dropping its proposed entry screening, following a consultation.
But Luton Borough Council complained to the adjudicator that the way Barnfield notified the public of its final determined arrangements meant it was too late for some people to give their views.
The council also objected to children of staff being given priority, but the adjudicator did not think this was “unfair or unreasonable”.
He said there would still be pupils who would miss out on a place even if children of staff were not given priority, because of the increasing numbers of pupils in the area.
But he did agree that the timings of Barnfield’s consultation had contravened the Schools Admissions Code.
Luton Borough Council’s overview and scrutiny board will discuss the judgement at its meeting tonight (Thursday October 11).
Barnfield assistant principal, Stephen Hall, said: “We are pleased that the Schools Adjudicator has rejected the local authority objections relating to the federation’s determining of its admissions policy, and confirmed that the plans to introduce the priority of children of staff were reasonable.
“In response to the concerns ‘partially upheld’ we have agreed to publish and communicate future determined admissions more widely than that undertaken.”