There's not really a lot of choices

Ruby's choices in the Lost & Found were not good.

There were certainly a few lost items such as notebooks, pencils, and even a back pack. But her choice of clothes were limited. Right now all she had managed to salvage was a white sports bra that was so tight and thin she might as well have painted it on her body. She had experimentally tried it on and was embarrassed at how tightly it had managed to conform to her body. Her raised nipples were easily visible. Honestly she'd probably be more decent being naked then wearing it.

She had also managed to find a pair of small red sport shorts, but those had come with the problem of being a size too small and also having a split in the back that would have shown her ass crack to everyone, the split itself no doubt drawing attention to her wardrobe malfunction and lack of underwear.

Ruby had honestly thought that finding clothes that actually fit her at the Lost & Found would have been the least of her worries, but right now her choices were so lewd she'd probably be better off just going naked since it would make her look less like an attention seeking slut.

"Professor Goodwitch, uh...are there any other clothes you could lend me?" Ruby asked. But turning around she saw only an empty hallway. The professor might have just thought she would manage to find a decent outfit and left to attend the speech by Headmaster Ozpin. She was the deputy headmistress after all.

Looking at the only clothing she had recovered, Ruby was left with a simple choice. Either she could wear clothes that accented her breasts and ass, only keeping her barely decent. Or try to sneak to her luggage in the ballroom on the other side of the school completely naked.