Inside Out by Maria V. Snyder

I'm Trella. I'm a scrub. A nobody. One of thousands who work the lower levels, keeping Inside clean for the Uppers. I've got one friend, do my job and try to avoid the Pop Cops. So what if I occasionally use the pipes to sneak around the Upper levels? The only neck at risk is my own. Until I accidentally start a rebellion, fall in love with an Upper and become the go–to girl to lead a revolution