S.D. Codified Laws § 13-7-34
If a school board has, by resolution, increased the length of terms from three to four years or decreased the length of terms from three to two years for the purpose of holding joint elections pursuant to § 13-7-10.3, the school board may decrease the length of terms from four years to three or increase the length of terms from two to three years using the same procedure as used when altering the length of terms for the purpose of holding joint elections pursuant to § 13-7-10.3, provided that each school board member is entitled to complete the term of office to which the member was elected. The school board is empowered to designate the number of vacancies and the number of the years, not to exceed three years, in each vacancy so that all succeeding regular elections have, insofar as practicable, the same number of vacancies to be filled.
SDCL 13-7-34