Harrasment, exploitation

In 2005, I had applied for the post of Catering Supervisor advertised scale (5300-150-7700-175-8450) in a prestigious school of New Delhi. The school follows central Govt. rules. But the existing catering supervisor did not want me to settle down, as I was highly qualified and suitably experienced, the existing catering supervisor was scared of me, out of jealousy he instructed the personnel officer to change my designation to store- keeper into a lower scale. By mistake after three months I had accepted the offer, presuming that it was the employer`s decision. But later I came to know that the employer (The Principal) was not aware of this, and everything was done by using the stamp signatures of the Principal. On the day of recruitment I was more qualified and experienced that was asked for in the advertisement. Now I want to fight against this injustice and want to get my dues back with retrospective effect. Kindly advise, how should I go about it ,keeping in view the prevailing circumstances, what are my chances?