A pupil may attend a school in an attendance district other than the one in which he or she resides with the written permission of the superintendent of schools. Beginning in the 2000-01 school year, the board shall provide spaces in each school for pupils who reside outside the attendance district for the school, but shall fill any unused spaces with pupils who reside in the attendance district. A pupil who attends a school may continue to attend that school until he or she graduates from the school and each sibling of that pupil shall be given priority over other pupils in the process of admission for that school.