If the residence of a pupil is so located that attendance in the district of residence constitutes an unusual or unreasonable transportation hardship because of natural barriers, travel time, or distance, the commissioner of education or his or her designee may assign the pupil to another district, except as provided in section 167.125. Subject to the provisions of this section, all existing assignments shall be reviewed prior to July 1, 1984, and from time to time thereafter, and may be continued or rescinded. Any assignment granted to a pupil under this section prior to August 28, 2018, shall remain in effect until the pupil completes his or her course of study in the receiving district or until the parent or guardian withdraws the pupil from the assignment. Any assignment granted to a pupil under this section prior to August 28, 2018, shall also be applicable to any sibling of the pupil and shall remain in effect until the pupil completes his or her course of study in the receiving district or until the parent or guardian withdraws the pupil from the assignment. The board of education of the district in which the pupil lives shall pay the tuition of the pupil assigned. The tuition shall not exceed the pro rata cost of instruction.