Fight for Life — Laurie Halse Anderson

Maggie has a mission

Maggie MacKenzie is great with dogs, but her homework is out of control. Dr. Mac, her veterinarian grandmother, puts her on a short leash until her grades improve. Four new volunteers show up to help Gran in the clinic, but none of them knows a boxer from a pug. When the clinic is flooded with sick and dying puppies, Maggie has to find a way to help, no matter what Gran says. Maggie is sure it can't be a coincidence -- somebody must be running a puppy mill. If she doesn't find it soon, more puppies will die!