Third Grade Baby

Now that Polly Peterson has a baby brother she's supposed to feel bigger, but her parents keep treating her like a toddler! And Zachary, the new kid, won't stop teasing her, just because she's the only third grader who hasn't lost a single baby tooth. But when her tooth finally falls out, Polly has a new problem. She’s not sure she can put her tooth under her pillow. Maybe third graders are too old to believe in the tooth fairy. If Polly still believes, does that make her a third grade baby?