SDCL § 9-7-3: Terms of office of trustees--Vacancy.

A member of the board of trustees holds office for a term of three years and until the member's successor is elected and qualified. A vacancy on the board must be filled as provided in § 9-13-14.1 or 9-13-14.2. A board of trustees may, by resolution, increase the length of terms from three to four years for the purpose of conducting the regular municipal election only in even-numbered years, as provided in § 9-13-1.3.

For purposes of staggering terms, when a municipality is organized, the trustees must be elected for terms of one, two, and three years at the first regular election. At subsequent elections each trustee must be elected for a term of three years.