Experience of music GCSE performance?

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My son is in year 10 so just started his GCSE's. Just after some experience of having gone through this, especially if you have a DC who is already above the required performance skill level.
DS has done grade 6 piano and due to sit grade 7 early next year.

It seems like the school is expecting him to perfect a piece every half term with about 4 weeks notice and no guidance on suitable pieces. He performed a piece that turned out to be grade 6 in difficulty, as he liked it, with 4 weeks notice that he was practising alongside other pieces for his grade 7. It sounded pretty good to me and his music teacher agreed that whilst it wasn't perfect at all, he plays it competently and pretty well considering the time he spent on it. But his school teacher marked him right down because it wasn't "performance ready". I get that he doesn't need to do anything above a grade 5, but this was his first assessment and who's to decide what is a grade 5 piece apart from if it's from an exam book.
I think next time I'm going to tell him to do a really easy piece grade 2 say, and get it perfect, is this the way to go? Also, just seems a waste of time every half term and gets in the way of prep for his grade 7. He does lots of extra curricular sport so time is limited and doesn't have the luxury of lots of practise time, also need to consider all the other homework and revision etc
Seems to me, if he needs to get a grade 5 piece perfect for the gcse at the end, he's already capable of it and can properly concentrate on just that for a month and get it done, but have to go through this every half term in the meanwhile.
Or am I understanding this wrong?

If I were you (him) I would dig out pieces he already knows well for these half termly things - pieces he actually did for his grade 5 for example. Free up his time for other things, and to pick the right piece for the actual assessments. It sounds like going easier but getting it spot on is the key.
Ive had one go through it but being honest it all got changed because he was a covid year so I don’t think my experiences would help!
Ive another in y11 now and never even had it mentioned to me so I guess he must be doing ok - oops! His report last week was fine so I’ll just keep quiet!

Tagging on to this, hoping someone can offer their opinion: My son mentioned that he’s already submitted his GCSE performance to school, which seems pretty early to me. He’s just rattled off a grade 6ish piano piece and the teacher told him it was fine, but I know it won’t get full marks as it won’t have been polished, and he’s bound to have rushed some of it. He says he can’t resubmit it, but this seems strange to me when it’s only November. It seems like throwing easy marks away if he’s not chosen something according to his strengths and the marking criteria. It’s OCR board. The school’s music results are okay but not great considering the cohort.

I don't know anything about OCR specifically but yes, November seems incredible early and he should be aiming to get the piece as good as possible.

It possibly depends on the school but your DS should be given opportunities to re-record it if he wants. We asked all ours to record a solo before half term and on an INSET day at the end of half term we marked them. Some are excellent and full marks so there is no point in re-doing them. Others would benefit from another chance. We give them ample time to record, we mark them, then let them know how they are doing so they can have another go.

Your DS's school should give them the raw marks before the 5th May so they can challenge the process if they want. We don't get challenges but we do get them asking for another chance, hence starting the process as early as possible in y11. Completing the paperwork is a complete faff so getting everything done early makes a massive difference.