My only problem with this is if it is getting State aid.
AFAIK, it is.
Christianity and affirmative action are incompatible.
I would assume this is a school run by either the Catholic or Anglican church and perhaps the board feels that its original purpose has been lost. There were some Catholic schools in the US that had so many non-Catholic students that they were in a similar position, although I think most of them have attempted to correct this now.
This is not a good idea. This will set a precedent. All communities will start to do the same and then you will see community schools and colleges for Brahmins, schools for other castes schools for Muslims - further ghettoization of people and then we will have more problems.
But hey, if the non-Christian staff of St. Stephens were also brought down to 50%, I am not sure it would be all that good a place to study in anyway.