I have an interest here too. However, I am not too sure of the chances of a successful appeal. On the one hand, although the school clearly hoped to have a bulge class next year, they were clear (on the open day I attended at least) that the necessary approvals were still pending. On a practical level, significant building works were required in order to accommodate the extra class. I doubt these can begin till the summer and the chances of reaching a quick legal ruling in your favour, including the time for inevitable appeals, poses a significant risk that they could be completed on time.