No warning is neccessary here for a foreign teacher unless you want to cheat him or her. Just give a notice to them and it will do. Foreign Teachers know who they are and where they have to go for the next step. As for me, I am happy every time I leave China to go back home except the matter of cheating me in making less payment made by the university, school or institute. They should be black listed in fact. But after leaving those schools, we don't bother them any more.