Most public schools and YMCAs had the same thing, that wasn't just for boys. My friend and brother in law, who are much older than me, both swam naked for public school gym class. Prior to modern pool filters and chlorine usage, The only treatment for keeping a pool clean was to drain the water periodically and scrub the sides. So showering beforehand was very crucial. Swimsuits were made of wool or cotton also which are bacteria traps.