The main school we send on to repeats the entirety of the Y6 Maths curriculum as Y7
Now I have opinions here. (Unexpected, and rare).
Cant and I have discussed this over many years and there will definitely be a KS2/3 transition huddle on the Enemies of Education Tour. But there is an assumption in this remark that repeating the Y6 work in Y7 is of course a pointless waste of time for those Y7s.
While lots of people are suggesting that secondary teachers visit primary schools to see the standard of work the kids are capable of, I think it would also be interesting for primary Y6 teachers to visit their classes in secondary school and see that they genuinely can't remember stuff they were taught in primary and need to do it again.
While I agree that Y7s should be exposed to new maths content to keep them engaged, they also need to repeat previous work. Maths is a spiral curriculum and they will be repeating Y7 work in later years in secondary too. We start Y9 the GCSE course with pen and paper addition, subtraction, multiplication and division.
Our Y7s recently did fraction arithmetic. They've done this in Y6. Did I teach fractions again to a class who were perfectly capable of doing it and were getting 100% on assessments before I'd even started? Of course not. Set 1 has the best hope of just needing a reminder although there will be some kids who still can't add fractions with different denominators or struggle with mixed numbers. Set 2 has some kids who will remember the method but still benefit from practice, and some who can do it after being retaught. Sets 3,4,5...by the time you get to set 5, you're back to 'colour in a third of this shape' in terms of where they need teaching.
Every year when I've taught a topic that kids have seen before I'll say 'You should remember this from Y8' and there will be kids who have sworn that they have never seen it before. At the start of my career I'd have wondered why the previous teacher skipped it, or maybe thought they were a bad teacher. That ended when one year I went 'Hang on, I was your teacher last year, and we definitely did this.
I also think transition from Y6 to Y7 probably gives them more time to forget stuff as the maths and English focus sharply drops from May.