The recommended formula for staffing an Australian school library is given in the standards of the Australian School Library Association/Australian Library and Information Association, Learning for the Future, 2nd ed. (Curriculum Corporation, 2001).
One day/week TL staffing is recommended when there are 3 teaching staff, up to one full time teacher librarian when there are 17 teaching staff, which would mean approximately 318 primary students. In secondary this would occur at approximately 221 students.
Two full time TLs are recommended for 50 staff, approximately 935 primary students or 650 secondary students.
The number of days of TL time would accrue according to number of staff/students. For details see the standards sample pages, including Table 6. If you know a teacher librarian, she/he should have a copy. Principals are urged to purchase these standards to guide development of their school libraries.
The NSW Department of Education staffing formula for teacher librarians provides an example of current policy.