A school board may establish a sick leave bank to enable a teacher, because of unusual circumstances, to draw not more than twice the number of days of sick leave the teacher has accumulated before the first day of school in any school year, or 24 days, whichever is greater. However, in a case of severe illness or extreme hardship the board may permit a teacher to draw more leave. The board may establish and administer the sick leave bank independently or jointly with teachers.