The Knave of Diamonds — Alex de Campi

Chloe's diamond necklace has been stolen by The Artful Dodger, and it will be up to Kat and Mouse to find it. However, this is no ordinary middle school mystery. To solve it, Kat must at last determine the Dodger's identity. But who is it? Is it the scheming Mimi? The misguided Ruth? The destructive Peter? The mysterious Mr. Templar? Or could it be someone else entirely? Whoever it is will undoubtedly face expulsion and probably find themselves in trouble with the law. Can Kat sentence a fellow student to such a fate? By the time this case is solved, Dover Academy will be shaken to its very core as the mystery of the school thief is finally solved!