I also assumed you were an assistant principal. In this case, you need to look at the job description for your position. If the things your principal said you need to do that require staying late are part of it, then fine. If not, it seems to me that she might be seeing you as an "assistant principal" who is there to take up the slack. Also, is the position "certificated" or "certificated management?" I've never heard of an instructional coach that was actually categorized as management. Coaches are generally "Teachers on Special Assignment" and have no supervisory duties. If that is the case, then you have contract with defined working hours.