An oversubscribed Catholic school has been told it can no longer give priority to children of parents who get involved in church activities.
A complaint against Coloma Convent Girls' School in Croydon, south London, was partially upheld by England's admissions watchdog and could have implications for other faith schools.
The school was told its points-based entry system was discriminatory.
This gave points for early baptisms and parents who help in church.
Coloma Convent Girls' School is heavily oversubscribed, extremely successful and graded outstanding by Ofsted.
The Office of the Schools Adjudicator's inquiry was triggered after complaints from a local parent and the school's religious authority, the Archdiocese of Southwark.
It argued that awarding more points for church activities was against its own diocesan guidance and could disadvantage certain groups, including single parent families.
And extra points for earlier baptisms was also against diocesan rules and could disadvantage others, including converts and those who had worked abroad.
The school said it had based its oversubscription criteria on date of baptism and church activities to avoid having one based on distance from the school.
This, it claimed, would have given an advantage to more affluent parents and worsened the ethnic and social mix of the school's intake.
However, the adjudicator found the school's intake was relatively less disadvantaged and had a lower proportion of ethnic minority pupils than its local area.
Only 3.5% of the school's pupils were on free school meals, compared with 14.4% at the nearest secondary school and a national average of 15.9%, schools adjudicator Dr Bryan Slater said.
'Clear code'
He also heard evidence from the diocese that parents in some parishes were deliberately carrying out church activities in order to gain points.
"In my meeting with the parties, the diocese stated that in one parish there were 100 children on a waiting list to be altar servers," said Dr Slater.
The Department for Education pointed to its admissions code, which sets out what admissions authorities cannot do when setting oversubscription criteria.
Paragraph 2.16c of the code says: "In setting oversubscription criteria admission authorities must not give priority to children according to their parents willingness to give practical support to the ethos of the school, which includes asking parents to commit themselves or their child to taking part in activities outside of normal school hours and asking parents to support the school financially or in any other practical way."
The code was strengthened in 2008, but a review of admissions procedures among the 3,500 schools which control their own admissions found half were breaking the rules.
The then chief schools' adjudicator, Sir Philip Hunter, said many schools were asking unfair questions on application forms such as parents' occupation.