Recovery of dues and experience letter from management

I was working with one institution as physical education teacher from the past three years.but this year in the month of April I resigned my service due to mental pressure and overload .i spoke to Principal of my institution for the same and submitted my resignation letter she has accepted .but now when I am asking my management to pay my deposited salary and issue me my experience certificate they denied and said pay three month salary or Serve three month notice period.my reason for resignation was the alloted me no of classes without any intimidation or prior discussion with me when I asked for the same our director repiled me that principal can allot any work to teacher is this the right.