Standard V. Program Management and Administration

Here is a video that very clearly explains the basics of Intellectual Freedom: https://www.youtube.com/watch?v=F8HMkHjBqJs 

I found these pages on ALA to be most useful:

American Library Association’s Code of Ethics  http://www.ala.org/advocacy/proethics/codeofethics/codeethics
American Library Association’s Intellectual Freedom resources  http://www.ala.org/advocacy/intfreedom
American Library Association’s Office of Intellectual Freedom  http://www.ala.org/offices/oif

This is the Texas Guidelines for School Library Programs
https://www.tsl.texas.gov/ld/schoollibs/sls/introduction.html