Mabel Makes The Grade (Sister Magic) by Anne Mazer

The first day of school is always Mabel's moment to shine. Nobody's ever more prepared than she is! But this year, things are going to be different. Her little sister, Violet, is starting kindergarten right down the hall. Mabel's pretty sure she can keep Violet's magic under wraps on the way to school. But what will happen during the school day? And how will Mabel stay a straight-A student if she's always keeping one eye on her sister?