There are going to be plenty of people out there who think you sit at your desk all day and do nothing, especially if you work in a school where you have a flex schedule. But truth be told you really have very little downtime, especially since those times you are not in front of a class you are writing lesson plans, answering reference questions, helping students find answers to questions and materials to use, and new technologies to implement in your school.
The job of a school librarian is changing, the school library goes far beyond the four wall of the actual room and it is our job to promote, and teach others just that. In my opinion, while books will never really disappear, electronic resources will continue to become more important. As librarians we need to be the driving force of technology in our schools, always on top of the new things out there, and that takes time. A lot of it! We need to search out new tools for teachers to use in the classroom that work seamlessly with their curriculum and enhance their lessons, while making there use easy for teachers. All this takes time, a lot of it. So be prepared to spend as much time on this aspect of the job, as on being in front of the classroom.