Wis. Stat. § 119.66: Interest in contracts forbidden.

During the term for which elected or appointed and for 2 years after the expiration of the term, no member of the board may be employed by the board or by the department of employee trust funds in any capacity for which a salary or emolument is provided by the board or the department of employee trust funds. No board member, superintendent of schools, assistant superintendent, other assistant, teacher or other employee of the board may have any interest in the purchase or sale of property by the city for the use or convenience of the schools. No contract made in violation of this section is valid. Any consideration paid by the city for a purchase or sale prohibited by this section may be recovered in an action at law in the name of the city. Any person violating this section shall be removed from any position held under this subchapter.