I was a piano teacher (I have recently stopped teaching due to a lack of time) myself. When I was learning piano when I was in secondary school, I had trouble understanding/remember all the details and concept.
The Grade 5 theory exam is difficult enough for a teenager, let alone a Primary 2 student. Personally, I refused to teach such a young child music theory as I simply had no confidence that he/she can pass given all the Italian musical terms and complicated concepts.
My two girls took the exam in Feb 2008 and pass the exam with 91 and 89. At that time, they were in P2 and P4. Actually, my younger one got the higher mark.
According to my piano teacher, if the child is good as maths then he/she will have an advantage. The teacher started to give them lesson in Aug 2007. She did give them exercise after each topics. From Christmas onwards, they started to do paper. They stopped doing papers for 2 weeks in Jan 2008 as it was their exam period. The week before the theory exam, I gave them 2 papers per day. Actually, I sat through all the lessons which helped to later on to correct their papers at home.
I was surprised to see their marks but later I found out that the exam of Spring 2008 was pretty easy.