I am not a lawyer, but I can read policy documents. WI statutes were revised to permit single-sex schools and courses. DPI has a very good question and answer page about the law and how DPI is interpreting it, at: http://dpi.wi.gov/sped/pnd-singlesex.html
The page begins:
On April 14, 2006, the Governor approved Act 346, which allows school boards and charter schools to establish single-sex schools and courses. Act 346 amended §§ 118.13(1), 118.40(4)(b)2, 119.04(1), and 119.22, Wisconsin Statutes, and created §§ 118.40(4)(c) and 120.13(38), Wisconsin Statutes. You can access Act 346 online at http://www.legis.state.wi.us/2005/data/acts/05Act346.pdf.