You can legally open carry with a CPL in a school.
However, if your school has a policy against employees carrying, then you cannot per their policy. While it wouldn't be a criminal act, it would be a violation of policy which could (and likely would) result in termination. DON'T DO IT.
Most employers out there have a no weapons policy for employees, including the parking lots. Some states have a law that says employers can't ban firearms that are locked in a motor vehicle in the parking lot, however, Michigan is not one of them.