In reality, at any school appeal, the school will offer reasons for a child not to be admitted. These reasons will include "the year group is full", "the school does not have the classroom space", etc, etc. Basically it is the job of the admissions authority (which can be the school or local authority, depending on the type of school) to prove that your child's admission to the school would be detrimental to the education of those already there. These things can be gone into quite deeply. For example, it is a legal requirement that each child has classroom space measuring x (not sure what it is). It may be that your child's admission would mean that the legal requirement was not met because the classes are small, etc, etc.
Your job is to prove that the school you are appealing for is the "only school" that is right for your child or that the extrance exam was not administered correctly or that there is other weighty evidence why your child should be admitted.
Having said all this (and it is info you already probably know) the appeals upheld and not upheld can vary from year to year because the school admissions number goes up or down or other possible reasons.
We appealed for a place for our third child at the grammar school where two siblings were in attendance. We had put the school as first choice and our child "passed" within the set overall passmark for the exam. We were able to provide excellent references from primary teacher and evidence that child 3 was at least as able as children 1 and 2. We were also able to give many other exellent reasons why we wanted our child to be admitted to the school. The result of our appeal .... "not upheld".
In fact, out of 40+ appeals for 2005 entrance, only one was upheld (I am aware of this persons grounds for appeal and suffice it say, I feel the panel did the right thing but I am not able to say what the grounds were, as I don't want to give anyone's personal circumstances away).
However, in previous years we had known several children to successfully appeal to the school. In some cases, the child had not reached the pass mark and in no case that we know, did anybody have really good reason to appeal (some had not even put the school as a first choice). The reason for their admission, and the non admission of our youngest child was simply that the school increased its admission number for 2005 entry by 10 pupils, thus making appeals less likely to succeed. In years previous to that, there had obviously always been room for a few more.
My point here, is that the appeals system suggests that each case is looked at on its own merit but our experience is that this is not the case. We do not take issue with our child not being offered a place at that school in the first place or with the fact that our appeal was not upheld. We do, however, feel that the system badly let us down because if other children were admitted in previous years "with less good reason" then our child should have been admitted.
In conclusion, there is some luck of the draw, which will vary from year to year, when it comes to appeals but the more evidence you can get, the more chance you have.
ps. One member of the Appeals Panel at our appeal said out loud that it was one of the best appeals he had ever seen!!