Congradulations to Peter. 82 marks is a good result at his young age, and I agree to Zoey saying that 其實比賽都係有點運數.
Regarding teachers, why don't you look around if you have doubts with your sons' existing teacher. Many teachers have trial lesson for you to observe his teaching method and to consider if he is suitable for your sons. I have friends who looks for different teachers every few years, or when the child reaches a high level likes grade 8 or the diploma. In APA, some serious playing students change to a more qualifying teacher at the high grades. There's no harm looking around while you consider.