Why not just use her socks?

She decided her best option at this point was to make a belt out of her bra. She could rip off the cups and shoulder straps, so only the band around her chest would remain. Then she could loop that through her skirt and hold it securely. She was going to have to get stark naked in the library for a while, but she would just have to deal with it.

She looked around, slipped off her shirt and unhooked her bra. She stood there in the suddenly chill air, wearing nothing but her socks.

Wait a minute, that’s it! Her socks! She could take them off and tie them around her waist like a belt! It would look pretty silly, but it would at least keep her covered until she could come up with a better solution.

She quickly slipped off her socks. She went to put on her bra next, but she realized she didn’t have time. It would be quicker to just throw her shirt on for now and put her bra on later when the rest of her was covered. She slipped her shirt on and hastily did up a few buttons, just enough to keep it closed. Then she set to work repairing her skirt.