Ok ... Are we talking about the mission from Cyclops Ave?
Are you sure you're in Ravenwood, Wizard City?
Are you sure it's the Myth school you're going into, and not the Myth tower? (School on the outer ring in Ravenwood; Tower on the inside ring.) Myth School is the one that, when you near the doorway, it says "School of Myth".
Do you have any connections problems? If you lose connection to a server, sometimes you'll see a room (or street) with nobody in it.
Have you knocked on the bathroom door just behind where he usually stands? Caution: He really hates when you do that, so you might better make that your last resort.
I am an IDIOT! I was following the yellow arrow and it was leading me to the Myth Tower - not the school. So once I realized what I was doing (a few hours after I posted this - I didn't realize what I was doing until I had to fight someone in my winter tower) I was able to fix the problem. Now I find him each and every time. I've been on a couple different quests since that one that I am supposed to talk to him and now the yellow arrow always leads me to the school...some sort of strange fluke (sp) I guess.