If you have a Diploma certificate then it is the equivalent for the 12th certificate. You didn't have to issue any other certificate. Now class 12th student have a more diverse level of options available, where as for Diploma students you are only eligible for the specific genre of course corresponding to your diploma.

To know more about the career after Polytechnic visit the link below:

https://www.careers360.com/careers/career-after-polytechnic

I hope this answer helps. All the very best for your future endeavors!