User blog comment:SpookyBoogie27/Light Novel Spoilers/@comment-174.119.241.202-20131221210446/@comment-16281343-20131222051514

I don't think they will graduate in Volume 19, not yet. Volume 20 or 21 perhaps. Since all the girls had their turns, the focus will return to Rias and that maybe her graduating and passing the responsibility to anyone in the ORC.