Ed. Code, § 18103

The libraries shall be open to the use of the teachers and the pupils of the school district during the schoolday. In addition, the libraries may be open at other hours, including evenings and Saturdays, as the governing board may determine. Libraries open to serve students during evening and Saturday hours shall be under the supervision of certificated personnel. Certificated personnel employed to perform full-time services in an elementary, junior high, or high school during the regular schoolday, may supervise, but shall not without their consent be required to supervise, a school library on evenings or Saturdays. If a person agrees to supervise the school library during Saturday or evening hours, he or she shall be compensated in the amounts determined by the governing board of the district as indicated on the salary schedule.