Meltdown — Chris Archer

In the final book of this middle grade sci-fi series, superpowered teens are on the verge of saving Earth from a mutant race when one of them disappears.
 
Toni Douglas feels like she's losing her mind. But maybe that's to be expected when you're a thirteen-year-old who has just traveled back in time to save the Earth. Somehow, just moments before she was about to stop a lifechanging meteor strike, she woke up in a strange bed. With parents she didn't recognize keeping a bedside vigil for someone named Denise Butler. When Toni tries to insist they have a case of mistaken identity, a doctor tells her she's suffering from amnesia!
 
But Toni knows exactly who she is. And if she doesn't escape this madhouse she's fallen into, she won't be able to find her friends and help them destroy the mutant Omegas who are trying to take over. So why does everyone keep insisting the Omegas -- and her friends -- don't exist? Somehow Toni has to convince everyone she's not crazy, and fast. The future of the world is at stake . . .