Wis. Stat. § 118.045: Commencement of school term.

(1) Except as provided in subs. (2) and (3), beginning in the year 2000, no public school may commence the school term until September 1.

(2) Subsection (1) does not prohibit a school board from doing any of the following:

(a) Holding athletic contests or practices before September 1.

(b) Scheduling in-service days or work days before September 1.

(c) Holding school year-round.

(3) A school board may commence the school term before September 1 in any school year if the school board requests the department to allow it to commence the school term before September 1 and the school board includes reasons with its request. The department may grant a request only if it determines that there are extraordinary reasons for granting it. The department shall promulgate rules to implement and administer this subsection.