Catholic pupils have crossed their own catchment area and attended the grammar as one of three "designated" schools for the past 25 years. Northfield and Linksfield, the two other secondaries involved, have never proved as attractive to parents.
Around 100 pupils are involved. Officials now want to reduce the extra numbers by upgrading religious education provision in up to six secondaries, or by making special provision at Harlaw Academy. Unpublished figures show a substantial number of Catholic children already attend local secondaries and not the grammar.
But Bishop Conti, while appreciating the council's difficulties, has condemned moves to disperse pupils. "We prefer designated schools because they can provide a learning community for Roman Catholic religious education, not just isolated lessons," he insists.
Mario Vicca, a leading lay spoksman and solicitor, accused officials at Monday's education committee of "unholy haste". Catholic pupils were not the cause of the problem and should not be the solution, Mr Vicca said.
He was referring to previous plans to rezone children from the Seafield and Viewfield areas away from the grammar. Following heated consultation, that plan was delayed for six years.
"What we have at present, which is educationally desirable, is a coherent group of young people who are aware of their identity and comfortable with provisions made for them," Mr Vicca said. "The importance of the situation at the grammar school to the Catholic community is beyond measure." It was essential for the small Catholic pupil population to maintain "a degree of association".
Councillors have now agreed to further informal consultation on a number of plans, including rezoning schools and cutting numbers of non-catchment area Catholic pupils. Doing nothing is not an option, officials maintain.
Pupil numbers at the grammar are already capped.