Why is faith considered more discriminatory than being a looked after child, having a sibling in the school, or distance?
Really??? You are really asking this?
Yes, the above are discriminatory. Of course they are. But most sane and compassionate people would realise that most looked after children have had a lot of disadvantage in their young lives and will continue to suffer that disadvantage. If they had to join a waiting list every time they were forced to move home and carer many would also receive no education. So a device that offers them an advantage above others has been introduced in order to counter the general disadvantage the suffer. I really don't think you can argue with that, it is entirely different from the Faith argument.
Siblings and distance from school - well, from a societal viewpoint it is best if siblings both go to one school and everyone has a chance to attend their closest school. Not only does it help build communities, not only does it assist parents to be economically active (it being hard to deliver to/collect from 3 different schools and still get to/from work on time - believe me, I have tried) it also will theoretically cut down on traffic pollution.
However, if we are being fair it would be best to bring in a lottery system. You apply to your preferred 5 schools, and schools select the children who are going to be given places randomly.
But anyway, even if this was the case - you still have the fact that Faith schools are allowed to discriminate on the basis of Faith, and I haven't yet heard a single good reason why they SHOULD be allowed to. If something is discriminatory surely it should have to have a very good reason to be? Not simply "well, we want to, so if you don't like it fuck off and live somewhere else"?