The Lions of Little Rock by Kristin Levine

As twelve-year-old Marlee starts middle school in 1958 Little Rock, it feels like her whole world is falling apart. Until she meets Liz, the new girl at school. Liz is everything Marlee wishes she could be: she's brave, brash and always knows the right thing to say. But when Liz leaves school without even a good-bye, the rumor is that Liz was caught passing for white. Marlee decides that doesn't matter. She just wants her friend back. And to stay friends, Marlee and Liz are even willing to take on segregation and the dangers their friendship could bring to both their families.