OBC Certificate

I am a government employee, when I joined service I was in class C my annual income was Rs 2 lakhs annually, then I got the promotion and now I am class B employee, annual income slightly more than 8 lakhs (so I am not a direct class B recruit). I know I am eligible for OBC NON CREAMY LAYER certificate but how will I get it for my child's admission in government institutions as states only have the principle of below 8 lakhs annual income. I don't have income from any other sources and my wife is a housewife.