Just to play devil's advocate here, it was my understanding that U of M is not a "school" as defined by law but rather a university. This happens for a few reasons, but the main reason is that the campus is so integrated into the city of Ann Arbor. For instance, there are HOMES (and private businesses for that matter) on the campus, and the University sure as hell can't deny those people the right to carry just because every time they step out their door they are "on campus". Whereas a "school" is usually a single building and property. Also, check your PM cgeorgan
EDIT: Did some research, while a "school" and "university" are different, Michigan in particular expressly prohibits carrying by permitted persons in college buildings. The law states that carrying is prohibited in "A dormitory or classroom of a community college, college, or university." This does NOT say on the grounds like OP is wanting to do. Although I can see it becoming a huge debacle if OP is caught, especially on the University of Michigan's campus.
Source: Chapter 28, Act 372 of 1927, Section 28.425o. Amended