Our pleated school skirts certainly weren't washable, so they went to the cleaners just once a term (if we were lucky
). Blazers? Never cleaned as far as I can remember - and yes, it had to last all my time at Grammar School too, as did the winter gabardine raincoat. This was halfway down my calves in the first year, but fashionably just above the knee by the time I left in the mid '60s!
School blouses changed twice weekly, cardigan (I only had one!) probably weekly.
As I recall, our PE kit (divided skirts, air-tex polo shirt and v-neck jumper) was hung on a hanger and kept in school, on our designated pegs in the cloakroom, so heaven only knows when that all got washed 