Long before Vatican II, Catholic sisters ran ballet schools.
For example, The Sacred Heart Sisters + Notre Dame Sisters in the US, UK, and France... SNIP
Ballet attire in class/stage = purpose: art + athleticism. Decency: covers what needs to be covered for that activity. Occasion: not a place of temptation.
Therefore: not sinful.
It IS sinful because it teaches little girls to use their bodies and think about their bodies in a way that is prideful and vain. If they want art, teach them embroidery and drawing. If they want athleticism, they can help with the housework.
The Psalms and other parts of the OT talk about dance in praise of God, not as in vain self-expression. Don't go twisting Aquinas to try to make excuses for vanity. Ballet isn't necessary to raise good daughters. It's a frivolous waste of time and money.
Still waiting for the exact url for where it says that Catholic sisters taught dance in the 1800s.