Babymouse by Jennifer L. Holm

Meet the incredible Babymouse! She is a young mouse with quite the imagination. In this first book, we meet Babymouse as she is under the nice warm blankets not wanting to get up to face the day. The first thing she faces when she gets to school is the locker monster. Her locker is stuck and nothing is going to change that. And the day just goes on from there. And then comes the news. Felicia Furrypaws is having a slumber party. Everyone is invited except -- you guessed it -- Babymouse. Will Babymouse find a way to wrangle an invitation to the party?