My other thread on here made me think about how common it is now to see the gingham summer dresses at school, as in my area they die out around p5 and certainly not secondary, even the private schools where there is very little between p4 and s6 uniform. DD is 11 and if she wasn't insisting on them I wouldn't bother getting them for her. BHS have them up to age 13 which I assumed was for larger than average primary girls, then the generous fit range takes them up to ladies size 20 or thereabouts. Do secondary girls wear these in some places? I did back in the 80s but this is 2013 where even 10 or 11 year olds rarely wear them.
Some year 6s still wear them. And some year 6s are pretty much their adult size.
Dd2 (year 4, but tall and slim/skinny) wears size 11-12 gingham dress because she likes it to be below her knees, and they're really designed for above the knee. SHe's about size 10 (child size) for trousers and tops. By year 6 I can see her needing bigger than age 13.
There are one or two private schools round here that go up to age 13 and summer dresses ar compulsary in the summer term.