Principal Harrasment

I am working in a private school as a primary school co-ordinator. I am with this school for about 7 years. Presently, since the administration staff resigned from her duties, the Principal asked me to take care of her activities too temporarily. Now, after 4-5 months I am involved in my regular teacher co-ordinator job apart from that front office handling, parents handling, admission & fee collections also sorry to say I started act if school-bell boy. I tried to take up this matter with Principal in-front of Chairman some days back. After that, the Principal of this school is a lady and she mentally torturing me stating that I am in-efficient and not capable of handling my job. I have tried to inform her that, these are not my duties for which she started behaving rudely. The school is holding my original documents and three months salary as a compensation if I put resignation. I am presently affected with mental as well as physical agony and had to consult a counsellor for treatment. I would like to seek your suggestion to come out of this trauma.