Now You See It, Now You Don't

There's a thief on the loose in the sixth grade, and Abby and Hannah are determined to find out who's been taking their friends' most prized possessions. They assemble a list of suspects and piece clues together just like real detectives. But when Abby discovers the identity of the thief by accident, she wishes she'd never set out to crack the case. Now that she knows what she knows, Abby has to make a tough decision. Should she squeal on one friend or sit back while her others get hurt?