Our (state) school does not stipulate which uniform is worn at which period. Hence you have boys who detest trousers wear shorts year-round and vice versa.
One of dd's friends detests the summer dresses so wears skirts and polo shirts year round, and another friend dislikes the winter uniform so wears the summer dress with tights and a cardigan in winter.
Ask your dc which they prefer and send them in that.
A summer dress for a state school does not cost a lot. And if you buy it big it will still fit at Easter.
My dd wore the same dresses last september and then again after Easter. She will go back to school in them this Sept as they still fit. (I bought a large size to get the length as I don't like school dresses above the knee)