I’d suggest that you ask the school directly if they ever take children out of catchment. If it is a successful, highly over-subscribed school with a clearly defined catchment area, pure academic achievement alone might not be accepted as a valid reason for appeal. Perhaps ask the school about that too. However, it really does depend on where you are and which school you applied for.
As we do not know the full circumstances, I would advise you to take some local advice rather than accept some ‘general’ help.