Legal Question in Constitutional Law in Wisconsin

That depends. Who are you? What is your relationship with the school? If you're a student at the school and it requires students to attend Mass, then you must either follow the rules or face the consequences those rules specify for failing to attend. Those consequences may include expulsion.

Private schools -- whether religious or not -- have contracts with the parents of their students. Those contracts oblige the schools to educate the students, but only if the students and their parents also honor the contracts. It sounds like this particular school has contracts which require students to attend Mass.

You posted your question under constitutional law. The Constitution limits only what the government can do. Catholic schools aren't part of the government, so they aren't subject to the same rules as public schools. A public school certainly could not require students to attend mass. But Catholic schools can, as long as they make that requirement clear ahead of time.