Promotion was revoked without assigning any reason what can be done

I am an employee of DPS RK Puram(School Id : 1719111) and my date of Joining in DPS RKP is 10th March 2005. I had been granted promotion in February 2015 along with 27 other admin staffs, as the same was reflected in my February 2015 salary slip. But promotion of 6 admin staff (including mine) was revoked and money recovered in the very next month i.e March 2015 without informing me of the reasons or the grounds for the revocation. I made so many representations to all concerned to know the criteria adopted, but all in vain. I happen to be the most educated out of all the 27 admin staff. My employer states that it was an inadvertent mistake. Kindly advise how can I fight against it? .