Being "advanced" is irrelevant.
What would be relevant is establishing if any appeal would be under ICS rules (ie you cannot have more than 30 pupils per teacher in years R-2). So do they combine year groups during those years, and how? If admitting more than 25 to reception means that at any point in those years, there would be more than 30, then it is ICS rules.
An ICS appeal can only be won on the following grounds:
a) the admissions arrangements breach the Admissions Code (eg not giving priority to a group they must, or giving priority to a group they must not) AND if the arrangements had been compliant, your DC would have been offered a place (This is very rare, BTW)
b) There was a mistake in processing your application (eg home to school distance inaccurate) AND if the correct distance had been used, your DC would have been offered a place
c) the decision is so unreasonable perverse it cannot be allowed to stand (bar for this is very high - eg placing a DC who uses a chair for mobility in the only school for miles around that is not adequately accessible)
If it is not an ICS appeal, then there is the further possibility of a "balance of prejudice" case - ie demonstrating that the detriment (ie prejudice) to your DC from not attending is greater than the prejudice to the school and its other pupils by going over numbers.
So does the school have unusually small classrooms that mean they cannot fit more pupils in them? Or is there another reason why its PAN is 25?
What benefit would your DC get from this school that cannot be had elsewhere? General statements about being "advanced" won't be sufficient, as (even if you think it's polite fiction) all schools are held to be capable of differentiating for pupils of all abilities. So what does this school offer that others nearby does not? Can you evidence (beyond parental assertion) that this provision is disproportionately important to your DC?
This part of the case would be important if the panel finds that the school can indeed admit up to a PAN of 30, but if there are 7 appellants for 5 places, they cannot all be admitted, so it will be the 5 for whom there is the strongest "balance of prejudice" case that would get offers.