User blog comment:Mnzombie/Issei's Peerage/@comment-24098614-20140110005926/@comment-1359322-20140110074131

They need to stay as Mid-class Devil for some time maybe 1/2 year to a year, it hasn't been long since Issei and the others become Middle-class Devil, so being High Class Devil during Rias and Akeno's graduation is a bit impossible.