mental harrassment

I have been working as a teacher in a private school for past 2 years and for the past 3 months my immediate superior is treating me so bad verbally after the school working hours as if I'm their slave. which took me to the state of depression and due to stress I became ill.. even at my sick leave.. they keep calling me from the institution for one thing or other and starts harrassing me mentally.. which pulls me into further depression. what action I can take so that I can be relieved from the institution since all my original certificates are surrendered there.