Thread:B214/@comment-27083298-20170505115317/@comment-29963364-20170507003020

Another thing you should consider about Rossweisse's teacher position at Kuoh Academy, is that she is Rias' servant, Rias' father is on the board of directors for the school, and they're Devils who can use magic to manipulate humans (as Rias did on Issei's parents when she first met them).

So it's absolutely plausible that Rias got a teaching position for Rossweisse despite her age possibley being at least 20-21.