Ann is like Eeyore, frequently blue and under the only raincloud around. But can you blame her? As the only scholarship student we know of in the story, she is kind of an island in a sea of gold. Mrs. Nightwing mentions that Ann is "One of our scholarship students," (4.64) implying there are others, but we never learn about any of them, so as far as the story is concerned, Ann is the only girl who is destined to become a nanny and servant instead of a wife and heiress to a fortune.
Ann holds onto hope that one day someone will rescue her from her miserable life—someone like Gemma's brother, Tom, perhaps—but Gemma thinks this hope is lame. No one like Tom will ever be reflective or insightful enough to realize that they could find a wife outside of the little bubble of wealth that they exist in, so Ann doesn't have much chance to get catapulted up to the upper class.
Ann's hope, though, provides an interesting—although small—tension to the book. While the rich girls are all trying to escape the endless list of boundaries and expectations put on them to make them "good wives" someday, Ann envies their position, while also being the only girl who has the potential to make her own life—and with it, her own rules—for herself. We're not saying Ann has it easier than her rich cohorts, just pointing out that there is a slim silver lining to her social positioning.
Ann is relentlessly careful, and if there are no rules, she needs someone to create them. This isn't really Ann's fault, though—as the product of a society that is inhumane and disdainful toward anyone who isn't part of the upper class, following rules is a way for her to safely navigate society. Her inclination to follow rules will help her land—and keep—a job down the road, most likely, and as a scholarship student, following the rules helps Ann keep her spot at Spence.
So when Ann is indecisive about joining the girls in their first night of drinking in the caves, it isn't just because she's never had a drink before—it's because the stakes are different for her as the scholarship kid. Gemma tells us:
Ann's eyes widen. The spoiled girls haven't any idea how agonizing it is for Ann to break the rules. They can always charm their way out of a certain amount of trouble, but for Ann, an infraction could be her undoing. (13.21)
Ann has a lot more to lose since she is expendable—not paying for tuition makes you easier to expel—but drink she does, since she also wants to fit in. This is what we'd call a measured risk, but what do you think?
No one is getting married yet (though Pippa gets super close), but Ann is just the sort to sing at other peoples' weddings: she isn't optimistic about her own prospects, she isn't very attractive with her constantly running nose and sourpuss face, and she will someday need to make a living by working. Singing is one of Ann's only positive attributes, and she knows it.
While in the realms, Ann's voice is supercharged, and she "is busy singing, gazing into the river, where she has assembled a make-believe audience of hundreds who clap and sigh and adore her" (25.96). Ann recognizes her singing voice as a strength of hers—perhaps as her only strength (she's filled with self-loathing, as evidenced by her cutting habit)—and relishes the chance in the realms to be appreciated for her talent. It's pretty sad when you think about it—Ann has to go to a magical world in order to be appreciated for something she can totally do in the real world.
And that's something Ann consistently reminds us about: the world is an unfriendly place to those who are different.