Aberdeen warns in its latest consultation on redrawing the grammar's catchment areas that in effect children would be able to claim they are discriminated against on religious grounds because they are non-Catholics.
"There should only be one primary criterion for determining priority admissions and that is address. The authority should not allow its duties to provide Roman Catholic religious education under the Education (Scotland) Act 1980 to get in the way of fulfilling its obligations under the Human Rights Act 1998," the council maintains.
The overcrowded secondary has 192 Catholic pupils and is one of three city schools offered to parents for denominatioal education. The other two nominated academies at Northfield and Linksfield are less popular.
To ease the pressures, the city wants to create a peripatetic service to support Catholic education in any secondary where parents want it. "Options have to be real, rather than, as at present, theoretical," the city says.
But a Church spokesman warned of continuing opposition. "It's an agreement that goes back 25 years and this seems a unilateral change. The proposals are inferior to the present arrangement," he said.
Any decisions depend on a formal consultation and final approval of the Education Minister. In a second and equally contentious part of the grammar debate, the city wants to press ahead with rezoning the Viewfield-Seafield area to Hazlehead Academy, a move hotly contested by parents.