@upsideup
You're talking about a crop top and high waisted skirt for a disco party. This is actually quite a stylish idea.
There's a vast difference between a chic outfit that is stylish but not just too much skin and two tint pieces of clothing that are tight and barely cover underwear. If the waist on a skirt is high style as you are describing and crop top sits a few cms above and isn't just a bra with a slightly different shape, that's your and her choice and it is even and location appropriate.
For sports, I wear small crop tops and yoga pants style tight leggings... to yoga! Yes completely appropriate for sports. If your daughter did gymnastics in a small leotard thingy at a class, that is sport appropriate, but would you let her walk home through town in what is essentially a swimsuit? It's location and activity appropriate.
I think the school are just trying to cover for all eventualities. Yes I think leggings or jeans and a modest crop top would be ok at that age for bowling if I was there, but short skit and crop top can range from classy, just, to practically just covering underwear. The school are responsible and will also be trying to teach various life skills. Like don't go out in public when we are responsible for you wearing clothes that are probably going to end up being trouble and please also learn that style says yes show some skin, but you don't need to show it all at once. Anna Winter, who they will never have heard of, would object to breaking basic style rules:) High waisted skirt and cropped top sounds pretty for a party.
Leggings or jeans and very modest crop top sounds ok for bowling, but if the school doesn't specify no short skirts or crop tops, chances are there could be kids in both a tiny skirt and tiny top. Bad style as an aside, but also not appropriate for location and activity. So they just set a simple rule.