It's entirely understandable why you want to resign as a staff governor, and you can do this at any time. You don't have to serve out your term of office. You send your formal resignation in writing to the Clerk to the Governors. You don't have to give any reason or explanation for your resignation, although I'm sure the chair of governors would appreciate it if you also called her/him informally to let them know why you are going. An election for a new staff governor should then be organised as soon as possible by the clerk, or whoever organises staff governor elections in your school.