The biggest differentiator between Grade 8 and an ARSM is the change in stamina requirement. This is not instantly visible to the candidate: it is a very different thing to play a programme lasting 30 minutes, than it is to play one lasting around 15 minutes, bookended with supporting tests. With this in mind, choice of repertoire is the foundation of good ARSM preparation.
Clearly, this is critical to the process, and has to be bespoke for each candidate. Each learners’ music tastes are different; I have pupils who would respond well to the challenge of learning some unaccompanied Bach, but who would resist any idea of playing a piece of, say, English music, with its sometimes rather coagulated harmonic language. On the other hand, I also have pupils who would love the latter and resist the former.
The beauty of the ARSM is that you can satisfy all candidates within a pretty wide-ranging list of choices. The challenge that all candidates will have to face at some point is the issue of pacing the programme. They will need to budget for how they will manage their stamina over that 30-minute performance.
One of the best examples of this kind of repertoire preparation is the issue of Baroque music. You can find examples of authentic performances on period instruments; and you can find examples of performances on modern/contemporary instruments in a more liberal style. I think it’s very important to encourage pupils to listen to both, and to make judgements about what the differences are. They can then decide how they plan to play, and make informed decisions about important issues like ornamentation.