What is the main idea to Wednesday Wars?

I think that different readers will see different main ideas/messages that this book presents. One major element of this story is the importance of family and friendship. Holling is not a super popular kid at school, but he does have a few key friendships. Additionally, his family life is not great. His mom and dad are not exactly the most loving parents out there, and his sister is cruel to Holling for most of the book. It is Holling's friends that see him through tough times with his family, but Holling never loses sight of the importance of family either. That's why he goes to great lengths to rescue Heather near the end of the story. That action restores his relationship with his sister.  
I think another key idea to this story is the importance that a teacher can play in a student's life. When Mrs. Baker and Holling start spending time together on Wednesdays, neither person is thrilled about the arrangement; however, over the course of the story, Holling and Mrs. Baker come to deeply respect and love each other. Holling would not have taken her advice about running, grammar, or acting had Mrs. Baker not shown that she cares for Holling. I think Mrs. Baker sends an important message to educators. Care for your students as people as well as learners.