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I'm looking for a novel in which the main character is a girl on a spaceship. In this spaceship, there are classes of people. The upper-class lives in the upper-levels of the spaceship, I think. The girl is a worker, the use tunnels/tubes/something like that to travel through the spaceship and she also hides in them sometimes. I think the people onboard are waiting to land on Earth? And they have to wait for a thousand days or something - they sing a song/recite a poem about that. The girl has a best friend who is quite popular among the workers. They were raised together by a nurse, like every children who is meant to be a worker. I'm not sure but I think at one point she hides someone in a sort of utility room and twins help her make a device. Maybe she goes to the upper levels of the building and meets a boy in one of the rooms and they later fall in love?
I think at the of the book, someone jumps/is thrown outside of the spaceship, and I'm pretty sure the girl's best friend dies. Or maybe he's the one who gets thrown outside?
(It's not Across the Universe by Beth Revis, though some things are similar - the atmosphere, especially.)
Any idea?

Maybe Glow?

It's not Glow, but I'll put that book on my list of books to read! ^^

Pretty sure it's Inside Out by Maria V Snyder. Sorry can't link from my phone. There is also a sequel Outside In. They are pretty good books:)

sounds like inside out to me, too.

Inside Out

definitely inside out and the sequel outside in which I finished an hour ago

You're right, it's Inside Out. Thank you a lot! ^^

Aww... and I totally knew the right answer! Glad you found it!