Hello School! by Priscilla Burris is a book told in short sentences and dialogue, making it a great book to start the year with. It gives you jumping off points to go over your own school and classroom rules. In fact, you could read the book as-is, and then go over your specific rules in the same way - by giving the rules and letting students speak up and share thoughts or examples, as they do in the story.
Butterflies on the First Day of School by Annie Silvestro, illustrated by Dream Chen, is a beautiful book about Rosie, a girl so excited to start school that she practices how she will act before the day comes. The morning of her first day of school, Rosie feels sick, and doesn't want to go to school anymore. Her mother assures Rosie that it's just butterflies in her belly, so Rosie gets on the school bus. Soon a girl sits next to her and starts talking, and when Rosie introduces herself, she's surprised to see butterflies escape from her mouth! This happens throughout the day, and when Rosie sees it happen to another girl, she knows just how to reassure her new friend. This is a fun way to talk about being nervous for the first day of school, but could also branch into a brief lesson about figurative language, depending on the age of your students.
The Class by Boni Ashburn and Kimberly Gee follows twenty children on their first morning of school. It shows how different everyone is, from how they get dressed to what they eat for breakfast. They all come together in their classroom, and this is a great jumping off point to let students talk about their first morning of school. Learning how different everyone's lives are at home can help these students practice empathy and understanding, as well as get to know their new classmates.
A New School Year: Stories in Six Voices by Sally Derby, illustrated by Mika Song, is like a version of The Class for older students. The six voices are students in kindergarten through fifth grade, and their stories are told in poems about The Night Before, In the Morning, At School, and After School. The characters are diverse and have different living situations, so this is another great book for students to read to hit home the point that everyone is different, and you should be kind and understanding to them. I like making copies of the poems and asking students to read them aloud. This can work in different ways - having students read one character's poems aloud in chronological order, or having students read the points of view of all different students according to each time frame. (I used it in different ways last year and had great results!)