Hydrochromia and light
[S3 Physics - Light] Ever walked in the rain, only to find that the thin, wet shirt stuck to your skin? Kind of a bummer that the older white uniforms tend to do that during rain, isn't it? It's particularly troublesome for the girls who have to deal with glares from other people, from the judgmental looks of elderly to the boys making a racket like monkeys. (Really, they should instead offer a jacket to help cover you up for warmth.)
Nowadays, most uniforms were designed with a thicker cloth material that's quite comfortable, and when those get wet it's not see-through. But why does that happen? Those shirts were supposed to be white and help keep your appearance orderly, lightweight and comfortable in the hot weather.
There you have it. Water contributes to the refractive index of a material as well, therefore when thin cloth gets wet, it sticks to your skin and appears see-through. It's useful in some scenarios, but can also be harmful in other cases. The more you know.