It is important that your school library have a selection policy in place. This not only helps guide teacher librarians and selectors in developing the library's collection but also lets others in the school community know the criteria and reasoning for the purchasing of materials, and how they support the curriculum and student learning. A selection policy is also of utmost importance if there is ever a challenge to materials in your collection. (See the reconsideration policy page.)
Resources and sample selection policies and criteria:
A Content Analysis of District School Library Selection Policies in the United States (PDF file from School Library Research)
District Selection Policies from the California Department of Education
Intellectual Freedom Resources from the California School Library Association
Sample Selection Policy from Santa Clara County Office of Education
Selection and Reconsideration Policy Toolkit from the American Library Association
(from the American Library Association's Selection Criteria webpage)