Validity of appointment done without framing or modifying the existing recruitment rules after upgradation of the post

I am working as a special educator in a state govt. school for the disabled that was upgraded from high school to higher secondary school(12th standard) in 2009. But the recruitment rules for the post of headmaster was not framed or modifyed the earlier R/R MEANT FOR THE POST OF HEADMASTER High school stage. And a person was promoted to that post on the basis of the R/R of Headmaster of High school stage in 2011. The post of Headmaster of this special school(Higher Secondary stage) is same in all aspect as the post of Headmaster of General Higher Secondary School but even not followed the R/R of Headmaster of General Higher secondary school. Further, the person appointed, though trained in this field but not registered, that is mandatory as per the Rehabilitation Council of India Act 1992. I have all the qualifications & registration as required but denied on the basis of seniority list. On which legal ground I may file a Writ Petition? Please help.