YANBU OP.
I asked for clarification about this regarding my DS (6). He has always been extremely ahead in Maths, he can easily compete Maths tasks for 2 school years ahead. (DH teaches Year 3, so was able to check this with resources he uses for his own class). He self-taught himself all his own times tables.
Fortunately, his school already streams, and he is sat with a table of other high attainers. As a child who was identified as gifted academically, I thrived in top sets which could move at a faster pace, mixed ability usually means for high attainers letting friends copy their work and explaining things to them. I actually never minded helping friends, but didn’t get much challenge myself. Sadly, this often doesn’t fully come until sixth form/university.
His teacher reassured me about the ways she is both deepening his knowledge and extending him. He has to show her multiple ways of solving a problem, and describe them. His table have access to ‘Power Maths’ books for extra challenges.
I was never as concerned about writing, as it’s naturally more open-ended for self-extension, but his teacher has done an excellent job of extending his vocabulary, punctuation and editing skills. Reading is also good, as we can access a new book daily, which is great as he reads so fluently and understands the books. Very able children can move at a faster pace.
The teaching standards make it very clear that the needs of all learners, including high attainers, must be met. Check out Potential Plus for further advice and resources outside of school. Good luck!