You don't need evidence of general
academic ability. However, evidence of ability in maths would be relevant to the school's specialism.
Probably best not to use the term 'extenuating circumstances' because it usually refers to reasons for underperformance in the 11+. Perhaps 'personal circumstances'?
You should be doing two things at the appeal -
1. Asking the panel to take into account the reasons for late baptism.
2. Putting forward reasons for wanting a place at the school. Reasons could include:
b) Commitment to the faith. (I don't know what evidence you had to send in for the admissions process - a form signed by a priest? - Perhaps you could ask the priest to write a letter of support specifically for the appeal panel?)
c) Any other reasons. See http://www.elevenplusexams.co.uk/appeal ... -school#c2