For me, I noticed that nobles won't give me this quest unless I'm at least clan tier 2, I've only ever seen it trigger for Mimir or Amorcon. The student cannot be leading a party, and needs to be under 21ish I'm pretty sure. I'm guessing the quest giver needs to be an older clan member, I've had Raganvad, Asta and Vidar give me this quest for Mimir before. Obviously with those conditions the quest is incredibly rare, I usually try to get the Mimir quest done in a playthrough if I can, but sometimes the conditions just don't get met