Wisconsin’s compulsory attendance law applies until age 18. So no, your 18-year-old is not required by law to attend school. 18-year-olds who are not attending school are generally either high-school graduates or drop-outs.
However, if you are claiming that your child of any age is attending school, and you wish to graduate your child and award a diploma, that child must be included on your properly filed PI-1206 Homeschool Report.
What’s the conflict? Well, while the law may not require your 18 year old to attend school, most employers, the military, and almost all institutions of higher education require completion of four years of high school. So if your child turns 18 before completing four years of high school, you most likely want to ensure that they attend those important four years of high school, and ensure that you have accurate, official records of that attendance. The only way to prove that your Wisconsin homeschooler has completed those four years of high school is to have properly filed your PI-1206 Homeschool Report every year.
If you don’t file a PI-1206 Homeschool Report for four years of high school, and your child has not been awarded a diploma in your homeschool, your child could be considered a drop-out under Wisconsin Statute sec. 118.153(1)(b)