The situation that should unfold at the appeal is that the first half of the appeal is whether the school proves it case that to admit all the pupils would be prejudicial to the efficient running of the school. In theory it is each individual pupil that should be considered but the admission appeal code is very clear that the panel has to make a decision as to whether all the appealants can be admitted to the school at the end of part one,as well as satisfying themselves that the admission arrangements were correctly administered.
If this was for less than 10 pupils then it would not be an ICS Regs case but as soon as the number of appealants goes over 10 then in effect the decision is whether we can break the ICS Regs and of course the answer is no, so they cannot admit all the pupils. There is no ability to just admit 10 or 8 or 6 of the pupils, it is all or nothing.
The second part of the appeal will be for each appealant in turn to explain their own personal reasons why their child should be admitted. When the panel has heard all the cases they will make decisions on each case whether the prejudice to the pupil outways the prejudice to the school. They then have to decide whether they can actually admit all these appealants who meet this criteria or not. Again the ICS Regs will come into play here and in effect the maximum number the panel could admit would be 10.
That is why in your case there are two things that become important. By far the most important is the strength of your personal case. It has to be strong enough to outweigh the prejudice to the school. One way to improve the possibilities of meeting this threshold is to reduce the prejudice to the school to the minimum possible. Obviously to some extent the school and LA have already made the PAN of 20 look too low by the fact that there are more than 20 in 6 of the 7 year groups so that is a good start but the other thing to do is to try and undermine the case as much as possible.
So questions about how good the educational achievement is, even with more than 20 in the year groups is a suitable line of attack, plus trying to draw out what is the maximum there has been in a year group ( I suspect this could have been 30) and asking about any additional buildings in the school and other alterations to improve education. The LA will undoubtedly say that increasing pupils will increase the Health and Safety risk, so you should risk asking the question as to whether there have been any reportable accidents that could be related to overcrowding. I have yet to hear a case where the answer was yes but I suppose there can always be a first time.