Wis. Stat. § 120.01: Number of school board members.

In common or union high school districts, school boards shall be composed of the following number of members:

(1) A common school district operating elementary grades or a union high school district shall have 3 school board members, except that if such school district is coterminous with a town or has a population of 500 or more it may have 5 school board members.

(2) A common school district operating elementary and high school grades may have 3, 5, 7 or 9 school board members.

(3) A common or union high school district may have not exceeding 11 school board members if a plan of apportionment is established under s. 120.02 (2).