School's Admissions office ignoring me after accepting admission offer
There should be faculty members in the department you are to be enrolled in who are responsible for graduate student admissions and advising. These people should be able to tell you what's going on with respect to your case because, unlike undergraduate admissions, graduate admissions decisions in the US are made with the consultation of faculty members and not simply by a "central" graduate admissions office, which usually just acts as a clearinghouse for the applications.