For what it's worth, DS and DD attend a high achieving comp. 360 kids in the year, two parallel top sets (one in each of the two houses). Each top set has about 32 kids, so about 64 out of the 360 total.
DS got an 8b in his end of Year 9 exam (and was working at this level during the year). He stayed in the second set (no further maths exam, no stats exam, no maths challenge, no enrichment).
DD has just finished Year 8, and got an 8c. She will stay in the top set.
I guess their school has a lot of kids good at maths. I think DS is about middle ability for the second set, and is predicted an 8 at GCSE. I am a bit peeved he misses out, when perhaps in other years he might have made top set, but in a way, given the new exam is harder, at least he will solely be concentrating on that, so will hopefully get the grade predicted 
Good luck with your conversation with the school OP.