You just answered your question. Whatever her reasons, I doubt whatever she is doing at night would be approved by the school's public morals. And yet, she's still rude and inflexible in her own enforcement of such rules: they're good for the others, but not for her. The fact she's also established to be some rich kid who doesn't even need to work just adds insult to injury, but I guess I shouldn't be surprised that in Japan, like the rest of the world, rules don't apply to rich people as they do to us, mere mortals.